Here is a photo PostalReporter.com posted about 4 years ago.

Houston,Texas, Drive-In Post Office , 1951
Granville W. Elder, acting postmaster of the Houston, Texas, Post Office, first experimented with a drive-in for customers in late 1947 to expedite the handling of Christmas mail. Set up in the Sam Houston Coliseum, the drive-in was used by thousands of Houstonians and relieved traffic congestion near the downtown Post Office. In the 1950s, drive-ins – some of them seasonal – also popped up at offices in Illinois, California, and Maryland, but the idea was never broadly embraced.