Added MAXAMT, allows negative testing of SetExpressCheckout call

Based on documentation here:
https://developer.paypal.com/docs/classic/lifecycle/sb_error-conditions/

This allows setting MAXAMT in the SetExpressCheckout calls, required for negative testing. This then allows a sandbox account to respond with an error, when enabled in the sandbox profile. See the link above for a full example.

This is useful for testing different response scenarios.