I’d have to, O’Connells is worth visiting. They have an incredible inventory, both current season and NOS. I’m not sure about a visit to Mercer, they have there offices in Montana and production on the east coast. To the best of my knowledge they don’t have any ready made shirts.I was living in my home country of the UK for many years whilst I became enchanted by natural shoulder clothing and I had more than my fair share of customs duties, plus items shipped from the US at great expense then returned at further expense due to being the wrong size.
Personally if I ever get the chance to visit the US again I would visit the following as a priority: O'Connell's; Rancourt; Alden and Mercer. (Mercer I don't think actually have many shirts ready to wear but if you have any doubts about your exact size it would be a good place to confirm it plus to see some of their cloths and shirts in the flesh and to order some.) Between those 4 shops you could get an extensive collection of very tasteful, high quality, classic ivy clothing. I'm sure Brooks and Press are also worth visiting but for me the 4 retailers I have mentioned would definitely come first.
One other thing: I would recommend that you get Shetland sweaters direct from Scotland.
Brooks Brothers, IMO, really isn’t a Trad store anymore. Rancourt and Alden would be an interesting visit. If they’re like Allen Edmonds they may have an outlet store.
I can’t really comment on J Press and their current offerings.