That's why I'm asking here. None of the web sites discuss it although Amplifier says the have "customer service". I'd rather hear from someone with experience before I start calling them individually.
There are hundreds of fulfillment companies. Even us. We are a brand but do fulfillment for a couple of other brands. You'll get a better, more targeted response if you let us know what the top 3 to 5 companies you're looking at.
Returns should always come back to the production facility. If it's the client fault (ordered wrong style or size) they pay return shipping. If it's our fault we either pay return shipping or advise the client to keep the item and we'll send the corrected one. All depends on cost of shipping (both ways) vs.our cost to make the item.
Our return ratio is approximately 1 for every 500 shipments. So, returns are a pretty minor issue when it comes to fulfillment (and t-shirt sales in general). Much more important things to consider in fulfillment are production cost, production time, and shipping cost.