I had Eric cut off the top of each post because I wanted to use this decorative cap instead. Was it necessary? No. But I like it much better.
We attached the caps with some wood glue then shot in a few nails from the top.
We then glued and Kreg Jigged a couple 2 x 4's to attach the two posts.
One problem we ran into was that we discovered the mailboxes each had a one-inch deep ledge framing the base which prevented them from sitting flush onto the 2 x 4's. Apparently there's a proprietary adapter that's available for online order, but we didn't realize that until it was too late so we had to come up with an alternative. The best thing we found (that were the right thickness) were two stacked fence slats of treated wood which we cut to size. We attached these to the 2 x 4's and eventually fastened the mailboxes which then sat flush on top of the 2 x 4's.
Throughout the day, I became progressively worse at taking pictures (I blame the extra runs to Home Depot due to unforseen issues that we had to address).
After digging the post holes sufficiently deep, we placed a couple inches of gravel rock for drainage and then lowered the posts into place. Once we got it all leveled, we used half a bag of Quickcrete into each hole. We added water and filled the rest of the hole with dirt.
The last steps were attaching the mailboxes and adding the number plates. As you can see, three of the mailboxes match, but unfortunately, when the final couple went to buy the same mailbox, it was no longer available so they purchased the closest option they could find.