This is a simple module to accept variable amounts (e.g. payments for services, or donations) via Paypal. Multiple currencies are displayed to the user to chose from. A thank you page is displayed upon return.

Here is the info file:

name = Payment
description = Enables payment via Paypal
package =
version = "$Name$"

And here is the module:

// $Id$

// Copyright 2007 Khalid Baheyeldin

// Variable names
define('PAYMENT_EMAIL',          'payment_email');
define('PAYMENT_URL',            'payment_url');
define('PAYMENT_COMPLETE_URL',   'payment_complete_url');
define('PAYMENT_PAGE_HEADER',    'payment_page_header');
define('PAYMENT_PAGE_FOOTER',    'payment_page_footer');
define('PAYMENT_THANKS_TEXT',    'payment_thanks_text');

define('PAYMENT_URL_LIVE',     0);
define('PAYMENT_URL_TEST',     1);

function payment_urls() {
  return array(
    PAYMENT_URL_LIVE => 'https://www.paypal.com/cgi-bin/webscr',
    PAYMENT_URL_TEST => 'https://www.eliteweaver.co.uk/cgi-bin/webscr',
  );
}

function payment_currencies() {
  return array(
    'USD' => t('U.S. Dollar'),
    'CAD' => t('Canadian Dollar'),
    'EUR' => t('Euro'),
  );
}

function payment_menu($may_cache) {
  $items = array();
  if ($may_cache) {
    $items[] = array(
      'path'               => 'admin/settings/payment',
      'title'              => 'Payment',
      'description'        => 'Settings for Paypal payment.',
      'callback'           => 'drupal_get_form',
      'callback arguments' => array('payment_admin_settings'),
    );
    $items[] = array(
      'path'               => 'payment',
      'title'              => 'Payment',
      'callback'           => 'drupal_get_form',
      'callback arguments' => array('payment_form'),
      'type'               => MENU_NORMAL_ITEM,
      'access'             => TRUE,
    );
    $items[] = array(
      'path'               => 'payment/thanks',
      'title'              => 'Thank you',
      'callback'           => 'drupal_get_form',
      'callback arguments' => array('payment_thanks'),
    );
  }
  return $items;
}

function payment_admin_settings() {
  $form[PAYMENT_EMAIL] = array(
    '#type'          => 'textfield',
    '#title'         => t('Email'),
    '#description'   => t('Email address to send payments to.'),
    '#default_value' => variable_get(PAYMENT_EMAIL, 'paypal@2bits.com'),
    '#size'          => 40,
    '#required'      => TRUE,
    '#maxlength'     => 255,
  );
  $form[PAYMENT_URL] = array(
    '#type'          => 'select',
    '#title'         => t('Payment URL'),
    '#default_value' => variable_get(PAYMENT_URL, PAYMENT_URL_LIVE),
    '#options'       => payment_urls(),
    '#description'   => t('The Paypal URL to redirect to for payment.'),
  );
  $form[PAYMENT_COMPLETE_URL] = array(
    '#type'          => 'textfield',
    '#title'         => t('Payment complete URL'),
    '#description'   => t('The URL on your site that the user will be redirected to after they complete payment.'),
    '#default_value' => variable_get(PAYMENT_COMPLETE_URL, 'payment/thanks'),
    '#size'          => 40,
    '#required'      => TRUE,
    '#maxlength'     => 255,
  );
  $form[PAYMENT_PAGE_HEADER] = array(
    '#type'          => 'textarea',
    '#title'         => t('Payment page header'),
    '#description'   => t('HTML text to display before the form.'),
    '#default_value' => variable_get(PAYMENT_PAGE_HEADER, '<p>Please complete the form below and then you will be directed to Paypal for completing the payment.</p>'),
  );
  $form[PAYMENT_PAGE_FOOTER] = array(
    '#type'          => 'textarea',
    '#title'         => t('Payment page footer'),
    '#description'   => t('HTML text to display after the form.'),
    '#default_value' => variable_get(PAYMENT_PAGE_FOOTER, '<p>We thank you for your business.</p>'),
  );
  $form[PAYMENT_THANKS_TEXT] = array(
    '#type'          => 'textarea',
    '#title'         => t('Thanks page'),
    '#description'   => t('HTML text to display on the thank you page.'),
    '#default_value' => variable_get(PAYMENT_THANKS_TEXT, '<p>Thank you for your payment.</p>'),
  );
  return system_settings_form($form);
}

function payment_form() {
  $urls = payment_urls();
  $url = $urls[variable_get(PAYMENT_URL, PAYMENT_URL_LIVE)];

  $form['#action'] = $url;
  $form[PAYMENT_PAGE_HEADER] = array(
    '#value' => variable_get(PAYMENT_PAGE_HEADER, '<p>Please complete the form below and then you will be directed to Paypal for completing the payment.</p>'),
  );
  $form['currency_code'] = array(
    '#title'       => t('Currency'),
    '#type'        => 'select',
    '#name'        => 'currency_code',
    '#options'     => payment_currencies(),
    '#description' => t('We accept payments in these currencies.'),
  );
  $form['amount'] = array(
    '#type'        => 'textfield',
    '#title'       => t('Amount'),
    '#description' => t('Enter the amount you are paying.'),
    '#size'        => 12,
    '#maxlength'   => 12,
    '#required'    => TRUE,
    '#name'        => 'amount',
  );
  $form['business'] = array(
    '#type'  => 'hidden',
    '#name'  => 'business',
    '#value' => variable_get(PAYMENT_EMAIL, 'paypal@2bits.com'),
  );
  $form['cmd'] = array(
    '#type'  => 'hidden',
    '#value' => '_xclick',
    '#name'  => 'cmd',
  );
  $form['item_name'] = array(
    '#type'  => 'hidden',
    '#value' => 'Payment for services',
    '#name'  => 'item_name',
  );
  $form['no_shipping'] = array(
    '#type'  => 'hidden',
    '#value' => 1,
    '#name'  => 'no_shipping',
  );
  $form['return'] = array(
    '#type'  => 'hidden',
    '#value' => url(variable_get(PAYMENT_COMPLETE_URL, 'payment/thanks'), NULL, NULL, TRUE),
    '#name'  => 'return',
  );
  $form['submit'] = array(
    '#type'  => 'submit',
    '#value' => t('Pay'),
    '#name'  => 'submit',
  );
  $form[PAYMENT_PAGE_FOOTER] = array(
    '#value' => variable_get(PAYMENT_PAGE_FOOTER, '<p>Thank you for your business.</p>'),
  );

  return $form;
}

function payment_thanks() {
  $form[PAYMENT_THANKS] = array(
    '#value' => variable_get(PAYMENT_THANKS_TEXT, '<p>Thank you for your payment.</p>'),
  );
  return $form;
}

The PAYMENT_URL_TEST => 'https://www.eliteweaver.co.uk/cgi-bin/webscr', is for checking the payment in the test account.