I believe I've found a bug (or, at least, possibly a minor feature request).
We have a simple donation form set up. For the "Pricing" fields, we have the following setup:
1) An "Option" field with "Radio Buttons" chosen as the field type. That "Option" field has 5 choices. The first 4 are pre-defined options with prices set. The 5th one has the label of "Other" and has no price defined.
2) A "Product" field with "User Defined Price" as the field type (to act as the value for the "Other" radio button).
If a user selects one of the first 4 radio buttons, the form submits just fine to PayPal. However, if they select the "Other" radio button, enter a value in the "User Defined Price" field and submit the form, the form never goes through to PayPal. Instead, it just shows the confirmation message and stores the entry.
I found the problem causing this issue, and figured out the workaround: If the "Price" is empty for any of the options, selecting that option will cause this issue, no matter what other information is provided in the form. Instead, you have to set the price to $0 for those options. Then, the form will work fine.
It would be nice if leaving the "Price" field empty caused the PayPal Add-On to automatically interpret its price as $0 (or, at the very least, showed an error message when saving the form, indicating that the "Price" can't be empty). Thanks.