I have the same problem, even if several months later. I've Isolated it to a specific issue.
I have two quantity fields, and a matching price for each quantity. I'm using PayPal Standard as the payment method. Each quantity field defaults to zero, and the user puts in the quantity they want for each.
If there is a quantity (not zero) in BOTH quantity fields, it extends properly and brings up the PayPal screen correctly, and once logged in, it's for the correct amount.
If EITHER quantity is zero, then this message appears. Problem has to be something to do with doing the extension and how the number is passed to PayPal. I would have thought zero is a valid number?
Any thoughts?
I have opened a ticket with MachForm, but no response yet. If I get one, I'll pass it along...
Conrad K.