90+ is too hot
If it is so hot that you are cautioned against leaving pets and kids in a car, then the postal truck may be too hot also. OTOH, wrapped in wet newsprint, double bagged, and in a box, if the postal truck does not sit closed up in the sun for long, and your mail box is not in the sun, you might be OK. Kinda depends on how fragile the plants are and how far they come and how much you spent on them.
This is when it is nice to have a local club to trade plants with, or a local auction to go to.
How far are you from Houston? The HAS auction is July 10th, Sunday, on the south side of town at the VFW hall across from Ellington Field, just east of I-45 and just south of Beltway 8. Auction typically runs from 10 or 11 AM to 6 PM.
How far from Ft Worth? The ACA meeting is July 21 - 24 and during that the AGA will have a table of plants for sale, but not in the auction. I think the local club also has plants in the auction or for sale somewhere.