how to add "void after 90 days" to A/P checks?

When you mention adding this text, do you mean you want it to appear automatically when you create the check transaction, or are you looking for a way to get it to appear on the paper when you print it?

QuickBooks Online doesn't have a built-in feature to add permanent custom phrases to check templates. However, you can use the Memo field. Any text you type there will automatically print on the check's Memo line.

You can check out this article, which outlines the exact rules for how QuickBooks Online handles the Memo field across different types of transactions: When the memo field will and will not print.

If you have additional questions or concerns, you can leave a comment below.