One of our users is having trouble paying us through our 30$/month payment option. He is located in India and is seeing the above error message. He is attempting payment through his credit / debit cards. The code I'm using for the payment button is the code that Paypal has generated for me.
Unfortunately that error can be for several reasons - for example, if a currency symbol is part of the value and then passed to PayPal, you'll get that error. If it's working OK for your other customers and not for this particular one - there could be an account issue. You may want to contact Tech Support, note the link at the bottom of the page "Contact Support".
