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A previous invoice might have the letter A accidentally added after the transaction number, shef.
QuickBooks will recognize that it's part of your preferences and will use it in the succeeding transactions.
You can run a transaction list report to check when it started and remove the letter from the transaction. Here's how:
Follow the same steps for the rest of the transactions. Then, create a sample invoice to test if the letter is no longer there.
You can also reply below if you still need help with this. I'll be around to make sure we sort this out.