Payment for services- user-defined amount to pay?

Hi

My client is a lawyer who wants clients at his office to be able to pay – whatever the service is by credit card online (it really can be any amount so ideally user defines it) .

He does not want client to have to create an account. I guess they can check out as ‘guest’ but my question is: does your plugin aplow for client to define the amount he / she wants to be charged on the credit card?

Tks in advance,
Ana