Doctor's Buggy kit
The Rondell kit is completely lacking these suspension details, and after seeing the real life examples, I'm thinking the wheels from that kit would be too bulky as well. The farm wagons and other utility vehicles had substantial wheels, but the mail wagon wheels were rather slight and dainty. During my research this week, I happened upon a 1:12 scale Doctor's Buggy by Model Trailways. This can be pricey depending on the retailer, but I found a great deal on amazon for $50 plus free shipping.
It has the exact suspension I need and daintier wheels than the Rondell kit. Considering the time and effort I would expend to recreate these parts, the kit is worth it for those parts alone. The springs and axles are actually metal in this kit. The wheels will spin and turn, which is just fabulous! I think it should be fairly straightforward for me to extend the suspension front to back to accommodate a mail cabin that will likely be longer than the kit's buggy compartment.