Here's an example on how to get the Drupal User ID into the link to CCBill.

First, you need to log in to CCBill and get the the URL to a CCBill form you'd like to use. This will look something like this:
https://bill.ccbill.com/jpost/signup.cgi?&clientAccnum=111111&clientSubacc=0001&formName=111111-0003cc-1&language=English&allowedTypes=0000000445%3A840%2C0000000214%3A840%2C0000001098%3A840&subscriptionTypeId=0000000214%3A840

Then, you need to add the user id of the currently logged user to the end of that link, using the following format:
&drupalUserId=<?php print $GLOBALS['user']->uid; ?>

The final CCBill URL would be this:
https://bill.ccbill.com/jpost/signup.cgi?&clientAccnum=111111&clientSubacc=0001&formName=111111-0003cc-1&language=English&allowedTypes=0000000445%3A840%2C0000000214%3A840%2C0000001098%3A840&subscriptionTypeId=0000000214%3A840&drupalUserId=<?php print $GLOBALS['user']->uid; ?>

Then, you can use that URL in any link on the site. Here's an example with the link text "Link to CCBill Form":
<a href="https://bill.ccbill.com/jpost/signup.cgi?&clientAccnum=111111&clientSubacc=0001&formName=111111-0003cc-1&language=English&allowedTypes=0000000445%3A840%2C0000000214%3A840%2C0000001098%3A840&subscriptionTypeId=0000000214%3A840&drupalUserId=<?php print $GLOBALS['user']->uid; ?>">Link to CCBill Form</a>

Then, you take that link and put it on a page. On that page, it's important that you set the input format of the page to "PHP code", otherwise the PHP won't be evaluated. One caveat is that this link should only be visible to logged-in users. There are a number of ways to make this happen. Here's one, this one also requires the input format of the page (or node) to be set to "PHP code":

<?php if ($GLOBALS['user']->uid): ?>
<a href="https://bill.ccbill.com/jpost/signup.cgi?&clientAccnum=111111&clientSubacc=0001&formName=111111-0003cc-1&language=English&allowedTypes=0000000445%3A840%2C0000000214%3A840%2C0000001098%3A840&subscriptionTypeId=0000000214%3A840&drupalUserId=<?php print $GLOBALS['user']->uid; ?>">Link to CCBill Form</a>
<?php else: ?>
You need to be logged-in to use this link.
<?php endif; ?>