I think I found a recent bug that is making testing checkout really difficult. Today on 02/04/2022 I started getting this 429 error when attempting to complete checkout.
It started off as very random occurrences, sometimes checkout completed fine and sometimes I got this error.
Over the last couple of hours all I get is the error after clicking on the Place Order button. I also found that if you click on Place Order multiple times it will eventually allow the order to go through , but this makes testing a very long process because you have no idea when the transaction will go through.