I made this template long ago because of the rule. The post office people have similar ones too. First Class Letters can't be over 1/4" thick. It's been suggested to put Styrofoam worms or small pieces of folded bubble wrap inside the envelope to make the mailpiece 3/4" or more. It's worth all the trouble because Delivery Confirmation (19-cents on computer mail postage labels) is cheaper than insurance (something like about $1.75 minimum).

Robt.