Using PayPal Sandbox to test Subscriptions Management

Anyone have experience testing with PayPal Sandbox?  I'm trying to setup and test purchasing memberships.  I created a PayPal Sandbox developer account and added a test Business Account and a  test Personal Account.  I have the Dolphin Subscriptions Management module installed and have setup PayPal to use the test Business Account.  I tried to pay for a new membership (upgrade from Promotion) with the  test Personal Account  and got this error message... which makes no sense to me:

We are unable to modify your subscription. This may be because your existing subscription calls for no more payments to be made or because it has been cancelled or completed.

Hello

This module uses its own integration of PayPal payment provider. So, I recommend you to contact with the vendor of the  module directly. He should be able to help you. 

I initially tried contacting martinboi through his Harvest Media website... no response yet.  It's been my experience that I will not get a response from him for several days. I'd like to work on a solution as soon as possible.

ive tried that sandbox thing and it didnt work so i had to set a test for a $1.00 and then made my test.

You've offered a good alternate suggestion... thank you.  I was planning to do both... with your idea as a final test with real funds.  My thinking was that the Dolphin Subscriptions Management has an option for testing with PayPal Sandbox... a very useful option (when working) to ensure every aspect of paid subscriptions are working as expected and securely.  

I contacted the developer yesterday after trying to get the Sandbox feature to work (getting repeated failures). He responded today asking for access information so he may look into the problem. 

UPDATE... today, martinboi found time to fix his Subscriptions Management module (on my site) so PayPal Sandbox test payments appears to be working.  However I now notice some other issues such as Standard and Promotion memberships not being immediately updated to the paid membership level once a subscription has been successfully purchased (via Sandbox test)... but I'll start a new topic for this issue.

All issues fixed by martinboi today. PayPal Sandbox testing works *and* correctly subscribes members to my website.  A sincere "thank you" to martinboi for fixing these issues! 

Thanks epaulo for your patience in working these issues out.  Glad everything is working for you.

I found another bug in your module... appeared after your recent 'fix'.  I PM'd the details to you.  I guess we should have tested the last fix more thoroughly.  I am so looking forward to getting everything working correctly. 

Thanks for examining the module so intensely, helps us to improve it.  And your minor email bug is fixed.