Ages since I’ve used Paypal sandbox and don’t have fond memories of it.
Currently I tend to use Warrior+ for most of my products – not ideal as people end up on their mailing list as well. Or for some products I create a Paypal button and just make sure I copy & paste the code correctly.
I don’t know how many products you’ve got but if it’s not far into double digits then I’d suggest just going with the Paypal buttons and making sure that the cancel link is added as well as the paid link (which I tend to set up Sean style as a “please confirm your details” autoresponder signup)
Aweber has a “send a test” link below each email in the follow-up series. I’d have thought Get Response should have something similar. Not as easy as one click to send the whole series but do-able unless the follow ups are too numerous.