Hi all!
Zvia, buyers are only able to pay for their purchase order on Etsy if you use Direct Checkout. If you don't use Direct Checkout, they can't pay for their order. Feel free to send the PO number to wholesalesupport@etsy.com if your buyer is still confused so I can look further into this and better instruct you from there.
White Moon, hopefully this won't happen, but if the payment date passes, first try to call your buyer. If they don't respond in a reasonable time, you can send the PO number and how many times/ways you tried to contact them to wholesalesupport@etsy.com so I can intervene on your behalf. Keep in mind the system doesn't send automatic reminders. The only message the buyer gets about paying is when you invoice them. It's very possible the buyer might forget they have an open invoice, so we suggest trying to contact them through email or Convo and phone as a gentle reminder before escalating to us.
Hope that helps!