Every here is probably familiar with the little tabs on Belgian stamps issued between 1893 and 1912 that instructed the postal service to deliver the letter on Sunday or not.
If the poster felt that Sunday delivery violated their religious beliefs that could leave the tab on. If the letter would have been delivered on Sunday it would be held and delivered the following Monday. If the poster didn't mind, they could remove the bottom tab that says "Do not deliver on Sunday".
Ever wonder what philatelists at the time thought about these tabs? Apparently they didn't think much of them, at least in London. Here's a little new issue news from the November 1896 London Philatelist
BELGIUM.—We are informed that the perforation between the coupon and stamp of the Dominical stamps is to be suppressed; in future, instead of the coupon being torn off, this will have to be crossed through with pen and ink. This report, if true, seems, if possible, to add further absurdity to these Non-Sunday Delivery Stamps. An overwhelming proportion of all the letters despatched in the Belgian Post Office are necessarily delivered on week-days, and it seems incredible that if, say, 100 letters are posted by a firm on Monday morning, each stamp has to be pen-marked, in case it should not get delivered before the end of the week!