I think the key issue (assuming your scenario plays out) is when...

I think the key issue (assuming your scenario plays out) is when the customer is told that there will be a surcharge. A lot of APT's goodwill with its customers is based on transparency and ambushing customers with a retailer surcharge at checkout will backfire badly on both the retailer and APT. I wonder if APT has the power to force retailers to advertise plainly (well before browsing and checking out) that the option will incur a surcharge and that it's not APT's policy but the retailers.

IMO that's just too much trouble for both sides and hence why I don't think it's going to happen with the vast majority of merchants. You make a good point though.