Is there a payment processor for squarespace which allows you to adjust a payment each time you send the link?

Hey,

I am building a website for somebody and they have asked whether it is possible to take payments with the option to change this amount for each client they have? For example:

Client 1: total cost £300 Deposit: £150

first link payment amount: £150 - second link payment amount: £150

Client 2: total cost £100 Deposit: £50

first link payment amount: £50 - second link payment amount: £50

Not sure if I have explained this very well. I am sure PayPal gives this option, I was just wondering if theres a more user friendly way?

Thanks,

Sarah