Shipmygun.. Can someone make sure I'm not crazy?

Sold 2 items (handguns) through another site. Been trying to navigate shipping to the customers ffl since.

Shipmygun was suggested to me. I read the UPS rules. Okay great.

Shipmygun says I must drop off at a "customer center", UPS website says "Customers may drop off pre-labeled and packed packages at UPS Customer Centers or The UPS Store locations or schedule a pickup with UPS."

But before this, says: "Shippers with an approved Firearm Products agreement may provide a shipping label to customers who need to return Firearm Products to the shipper (or to another address as agreed upon between UPS and the shipper)."

So some questions:

- Is SMG the one with "an approved firearm product agreement" in this case and they are letting me use it essentially?

- UPS website says next day air services only but SMG says 2nd day air services. Which is it?

- SMG only has 3 box sizes listed, where am I supposed to find new boxes in their 3 obscure sizes?

Can someone walk me through it as if I'm a caveman?

Thanks all.