Taxes-calculation in /cart/checkout is not always the same

Hi everyone!

This bug is somehow related to #1178224: Payment-Tab not removed on order with value 0 at the uc_free_order module... But as of writing comment #1 there, I start to think, that the bug there is caused by a malfunction of the uc_taxes-module.

The situation (if you read the other bug and comment #1, you might as well skip that part):

The product I'll always use
-> Price: 14,90 Euro
-> Taxes: 19% (2,38 Euro)
-> Price without taxes: 12,52 Euro

uc_free_order uses

$('#edit-panes-payment-current-total').click(function() { free_order_check_total(this.value); });

to decide how much the value of an order is. This function is basically called every time /taxes/calculate returns new values.

When I open /cart/checkout, I'll get the following to results:
14.9000019
17.2789938

OK, the first one is the product with taxes, the second one seems to be price-with-taxes + taxes - that's completly senseless.

Now, I add an 100% rebate coupon. /taxes/calculate is - of course - called one again:
4.758993799999999
2.3800019

Note, that a line-item with -12,52€ / -2,38€ / -14,90€ (price with taxes / taxes / price without taxes) is created.

So as far as I understand, 4.758993799999999 is that "2 x taxes"-thing, then the tax-value of the coupon (egally 2,38 Euro) gets distraced. That leaves the value of #edit-panes-payment-current-total to 2,38 Euro, even tough in the payment preview-pane we have 0,00€ / 0,00€ / 0,00€.

That is the first kind of strange behavior, but there is more...

If I leave /cart/checkout and return, I still have my cart with my rebate-coupon and a total value of 0,00 Euro.

But in background, /taxes/calculate gets called either way.
0.0012018999999999
0.001201899999999867
2.3801938
0.001201899999999867
2.3801938
0.0012019000000002044

Well, I realy dont know, whats happening here and what gets calculated, but fact is, that the last result is a correct (rounded) 0,00 Euro you'd expect from a cart with a 100% rebate coupon.

To conclude after this long text (sorry guys): I thing there might be something wrong with the uc_taxes-module.