I wouldn't think so at this stage - I had a quick look at it and it seems it works the same way as most other shipping services, where the seller pays for postage and ships the package directly to the buyer via that service.
S2A are just using DHL, so they probably send stuff out by container loads to get slightly cheaper rates (some of my suppliers offer the two options for couriers like DHL - direct or via an agent, the latter is always cheaper and what S2A would count as, I'm guessing).
I looked up a small-sized, 250g parcel to the US and got a quote of $46 (they pick up from you, it seems, and average delivery time-frame was quoted as 4 days - cheaper than AP's express courier international, particularly as - if I read it correctly - insurance was included (up to $500, I think?), but probably only worthwhile if you're looking for alternatives to that).
