Payment solution

Firstly, I wasnt sure where to post this question as i have both eCart and Payment Buttons extension.

I need some advice on the best product.
I have a paypal business account.

I need to build a payment page where logged in users select a credit package from a simple drop down, where each option has a different cost value.

<select name="cred-option" id="cred-option">								        																        <option value="5.00">3</option>
<option value="8.00">7</option>
<option value="10.00">10</option>
<option value="12.50">15</option>															                                                                              </select>

then proceeds to purchase. No downloads of any description or carriage and tax etc required.

When a user selects an option to buy, I need the 'value' used in the payment transaction.

For example, if a logged in user selects option 7, they will need to pay GBP 8.00.

Both values need inserting into my transaction table thus;

the credit value 7 into the credit-qty field and the credit cost GBP 8.00 into the credit-cost field. I can sort any transaction id's and dates etc.

Any advise on simplest solution would be appreciated.

Thank you.