This bug report is over one year old and may not be relevant.
Clarification Requested (?)
Simple Commerce Module Does Not Work With PayPal Sandbox
When in Sandbox mode, PayPal does not return any data in the `custom` field, upon which the Simple Commerce module relies (it should include the member ID of the purchaser).
This may be a bug on PayPal’s side of things, or it could be an official change in the way the PayPal IPN functions which is not supported by the Simple Commerce module.
The official documentation makes no mention of the `custom` variable not being returned in Sandbox mode, so I suspect it may be a PayPal issue.
Either way, it’s a big problem, for two reasons:
1. It makes it impossible to test a Simple Commerce site without making actual purchases;
2. It’s not a documented issue, and is a complete pain to track down.
Finally, it’s worth noting that this may also affect non-Sandbox mode (I haven’t checked). I doubt it, as I couldn’t find any bug reports to that effect, and this would be a pretty major issue if it also affected live sites, but figured it was worth mentioning.