Did you follow their instructions to the letter?
Vendors like Amino Lair that are still able to accept payment methods like PayPal, Zelle, Venmo, etc. usually have a very specific process they require customers to follow regarding payment, and part of that usually includes acknowledging/confirming that you completely understand and will follow that process.
From what I can gather from your post, the breakdown appears to be understanding that only the word "thanks" and nothing else should appear in the memo field, and — perhaps most importantly — communicating that you understand that back to them.
If this had been me, I would have responded with something like "I understand, and I will only put the word "thanks" in the memo field and nothing else". I assume the next step would be for them to provide the payment address. It sounds like that may be the part you're having difficulty with and why they couldn't give you the payment address.