Every Door Direct Mail is great shop revenue — the customer buys printing and a mailing product. It's also a category where a sixteenth of an inch gets a finished job refused at the post office, with your customer standing there holding the boxes.
The flat rule
EDDM Retail pieces must qualify as USPS flats: the piece has to exceed 6.125" in height or 11.5" in length. Exceed — equal isn't enough, which is exactly how shops get burned: 6.125×9 fails; 6.25×9 passes and is the industry-standard EDDM postcard for that reason. The ceiling is 12×15".
The indicia
The mail panel needs the EDDM indicia — the permit box reading PRSRT STD / ECRWSS / EDDM Retail / Local Post Office — printed, not stickered on later in a panic. Build it into the art before plating; it's the single most forgotten element in EDDM prepress.
Bundles and paperwork
Pieces go to the post office in bundles of 50–100 with a facing slip on each, plus the route paperwork from USPS's EDDM mapping tool. Not prepress exactly — but the shop that hands the customer neat bundles with slips is the shop that gets the reorder.
Where automation earns its keep
The size check is pure rules, which means software should catch a non-compliant piece the moment the file lands — before design money is spent, before plates, and long before the counter. (Plateroom runs the flat check on any job with "EDDM" in the work order and flags the indicia reminder automatically. There's also a free size checker.)