In most cases, you are setting up products on sites to use a set pricing structure. It is possible to setup a product to accept a payment for any dollar and cents amount, with a creative use of the quantity column in a pricing calculator. The product name can be set to be edited so that the order title or subject can be customized. Screenshots for illustration and a download of the spreadsheet that you can use are shown below.
Pricing calculator engine for the online invoice payment. The spreadsheet can be downloaded here: OnlineInvoicePayment.xls
Quantity is set to have two decimal places, and allow arbitrary quantities is enabled.
Allow editing of the product name is enabled so that the order title can be customized.