What I've done:
I've went through a nicely compiled list to make sure I wasn't missing anything from this source: http://troybrant.net/blog/2010/01/invalid-product-ids/
Everything checks out except for one; The bundle ID. I'm trying to purchase from the iMessages "extension", It has a different bundle Id than the one on iTunes Connect, but I figured since it's an extension, it should find the IAPs.
I would appreciate any help. Thanks!
Thanks to Apple's great error messages, they were very descriptive when saying "Invalid Product Id". It turned out I didn't have my bank details active. Anyway, fixed that, and it solved my problem.