If you are interested in mail trains, there are many different sorts of cars in them. Following are some of your choices: Many are "price friendly"!
This boxcar can be used as a storage mail car (unopened until destination) or express shipments. Basic Athearn, in the ten buck range
Fresh fruits and flowers, especially the fist harvest often were shipped in express reefers like this one. It's an Intermountain model, in the thirty buck range. Kits are availavle on the swap meet circuit for less, say in the fifteen buck range:
Below are three different models of the same car, a SP 60 ft. baggage, similar car used by the UP. On top is the latest update of the old Athearn Roundhouse model. The roof is magnetically attached, its easy to add an interior. In the middle is a brass car, (don't ask how much that one cost!). I wanted it because it is correct. Note the panel count between the baggage doors. This is the only flaw in the Athearn/Roundhouse model. On the bottom is an old Roundhouse kit I upgraded with Central Valley trucks, Walthers diaphragms, Bethlehem car works steps and Walthers roof vents. I did a string of these
Below is roof detail for the three cars. The brass car is on top. Note the presence of conduit and the gas line on the brass model, one of the reasons you pay so much for these darned things! In the middle is the upgraded Roundhouse, and on the bottom is the New Roundhouse model. The globe vents here are really more for passenger cars than for baggage cars. Note the little vent on the roof. It's for a toilet. Some of these cars carried messengers, so they had a desk and "facilities" for the messenger. Those cars had a star above the number. The upgraded Roundhouse model is in the thirty buck range. the old Roundhouse are all over e-bay at various prices averaging around fifteen bucks
Last, the GGD models. Anyone hear of GGD? Golden Gate Depot. They are a division of Sunset Models, a brass importer, and they did a series of plastic Harriman cars. All of them had problems, some worse than others. The Baggage and RPO's were done as "shorties" They were up front about this, but still, I never got why they did that. Note the difference in length against the correct brass car. they also left off some rivet detail on the sides and roofs. It was their first try at HO plastic, and they stumbled a bit. The coaches and the All Day Lunch care were OK. All cars were lit and had good trucks, diaphragms, and overall nice detail. At $90.00 a pop though, that's too many issues for me. That being said, they look ok in a train following the three foot rule. I guess most modelers didn't care as they sold very well.
Maybe next Trainfest I'll display a mail train!