Most of the USPS last mile delivery trucks are a custom vehicle called the Grumman LLV, with mechanicals based on the S-10 small pickup of the era (late 1980s to early 1990s). A small number of newer minivans seem to show up, probably as some of the LLVs become unusable for whatever reason, though their interior and cargo arrangements are probably not optimal for mail delivery (though mail carriers in warmer climates probably appreciate the presence of air conditioning). Note that the LLV is right hand drive, the opposite of most vehicles in the US, and has a sliding driver's door. A regular Model Y would not be anywhere close to the optimal mail delivery truck.