Complaint Review: Sensa Weight Loss System

Jan 24: I signed up at TrySensa.com for a "free trial" of their weight loss product.
There policy is you have 30 days to try it, if you don't like it, go online and cancel your membership, they issue and RMA # and you send back any unused portion. If you don't, you automatically get the next shipment and they charge your card $89.95.

Feb 23: the product made me nauseous, so  I filled out the form, got my RMA, and (Mar 10) went to the post office and mailed it back in a medium sized envelope. 

May 24: I get a call from a Collection agency claiming I owe Sensa $89.95 because they never received my return. They give me the Sensa phone # and I call them.

After about 30 minutes of arguing with the rep, she tells me they have no record of my return and I have to take it up with the collection agency, and there is nothing they can do. I ask them what on earth the collection agency could do, and she basically told me "that's for you to figure out" She then proceeds to "forward" my call to the collection agency, which ends up being the number for a tavern.

All I have for proof I sent it is a Paypal transaction (i used my Paypal debit card at the post office) for USPS, but it doesn't have any details of what was sent or who it was sent to. The only other issue is I may have forgotten to put the RMA# on the envelope. Though it was addressed with my name.

Every interaction I have had with any customer service is "you can't prove that you sent it, so we have the right to charge you for it"