The most expensive mistake in contractor direct mail is a design decision that nobody realises is a postage decision. The Domestic Mail Manual sets the envelope for First-Class Mail card prices at not less than 3.5 inches high, 5 inches long and 0.007 inch thick, and not more than 4.25 inches high, 6 inches long or 0.016 inch thick. Step outside any one of those three and the DMM has a one-sentence answer: ineligible cards are subject to letter-size pricing. Nothing is refused, nothing is returned — the piece simply costs more, on every card in the run, and the first anyone hears of it is at the counter. This builder takes the width, height and thickness you are actually printing and tells you which side of that line you are on, with the section number, before the press runs.
The second trap is thickness, and it catches exactly the people who try to get clever with size. A letter-size piece has to be at least 0.007 inch thick — but if it is more than 6 inches long or more than 4.25 inches high, the minimum rises to 0.009 inch. A big 9 by 6 inch card printed on 100 lb text stock lands at about 0.007 inch and is not mailable at all. It is not a pricing problem at that point; it is a rejected mailing. The builder applies that clause the moment the piece grows past the card maximums, which is the only moment it matters. There is a shape rule too: DMM 101.1.2 makes a letter-size piece nonmachinable if its aspect ratio — length divided by height — is less than 1.3 or more than 2.5, which carries a surcharge. That section is written for letter-size pieces, so for a card-size piece this builder reports the ratio and says the verdict is not evaluated, rather than inventing one.
The third trap is EDDM, and it is the one that produces the most confident wrong answers online. EDDM-Retail lets you mail every address on a carrier route without buying a list, but it is a saturation FLATS product, and the Quick Service Guide is explicit: every piece must be at least 5 inches long, 3.5 inches high and 0.007 inch thick, AND must have at least one dimension greater than 10.5 inches long, 6.125 inches high, or 0.25 inch thick. Note the 10.5 inch figure — it is not the 11.5 inch First-Class letter maximum, because EDDM-Retail is USPS Marketing Mail and that is the threshold published for it. The practical consequence is blunt: a 9 by 6 inch postcard, which is what most people picture when they say "EDDM postcard", exceeds none of those three and cannot be mailed EDDM-Retail at any run size. A 6-1/2 by 11 inch card can. The maximum is 15 by 12 inches at 3.3 ounces, and each mailing runs 200 to 5000 pieces per five-digit ZIP per day.
The fourth thing this builder does that no postcard template does is show you how little of the address side is yours. The barcode clear zone is a rectangle in the lower right corner of the address side running 4.75 inches in from the right edge and five-eighths of an inch up from the bottom, and nothing may print in it. The recommended address placement sits between five-eighths and 2.75 inches above the bottom edge, half an inch in from each side. Postage takes the upper right corner — a meter indicium sits a quarter inch off the top and right edges with a half-inch clear zone to its left and below. Add those up on a 6 by 4 inch card and the message column you have left is a little over an inch wide. The builder computes all four rectangles for your actual trim, draws them on the exported PDF, and budgets your message copy against what survives, which is why the character count it gives you is smaller than you expected and correct.
What this builder deliberately does not do is quote you a price. Postage changes, and a template that hardcodes the current card rate is a template that lies to somebody next January. Every export says the postage price is not evaluated and points at Notice 123 and the Post Office. The same honesty runs through the advertising check: several states require a contractor licence number on all advertising, and a mailed postcard is advertising. California requires the number in all forms of advertising under Business and Professions Code section 7030.5. Florida requires it in each offer of services, proposal, bid, contract or advertisement regardless of medium. Texas requires an electrical contracting company's advertising to carry both the company name and the licence number. Those three are read and cited here. Every other state prints "not evaluated — check your state board", because there is no authoritative fifty-state table to copy and a wrong verdict on real advertising is worse than no verdict. The builder also checks that the number is assigned to a face at all, since a number sitting in a form field and not placed on the card does not print.
A plumbing postcard is kept on a fridge until the day it is needed, which is a different job from a flyer. Put the emergency number where a hand can find it without turning the card over, and put the licence number somewhere a reader can check. The card that gets kept is the one with a phone number big enough to read across a kitchen.