Zoho Online Payment Options

Hello,

Does anyone have experience setting up online payments in Zoho? Either through Zoho Books, Zoho Checkout, Zoho Sign, Zoho Forms, etc. Seems like there are a lot of options but I'm looking for something that will allow our customers to pay a dynamic value online based on the input of a field in Zoho CRM. Our service is commercial appraisals so every sale will be a different dollar amount and it seems like none of these options generate a dynamic value for the sale amount and want it hard coded.

For reference, these are inbound calls from people interested in getting an appraisal, they agree and are sent an engagement letter to sign, we would like for them to sign the engagement letter and pay right away. Any thoughts on this would be GREATLY appreciated!