Give TubeDepot a call. I have returned tubes to them with no hassle ...
"Items are ordered, received and used (with apparent use), then returned to us (Non-Defective Apparent Use Return), then the items will be sent back at the customer’s expense."
On returns they accept, there's a restock fee:
"The restock fee serves to insure that the product we restock meets our manufacturer's quality standards. Returned product is often sent back to the manufacturer for quality verification or replacement. The restock fee also gives our customers peace of mind that their tubes and / or parts have not been 'tried out' by a previous customer.