How do you abbreviate apiece?

"A piece" is already an abbreviation of "a piece" and there's no shorter form. You can use "ea." which is short for "each" when writing a list of items, but it's not quite the same as "apiece."

Example:

* Incorrect: 3 apples ap.

* Correct: 3 apples ea.

It's best to avoid using abbreviations unless they're widely understood and appropriate for the context.