@Bill *********
There was a sure misunderstanding between you and the check-in clerk
***you won’t have to pay 20,000 THB to Immigration on your return
*** your other misunderstanding is that you assume you were issued a 60 days tourist visa on your second entry. NO, you entered WITHOUT a visa, which is a “visa-exempt entry” and you received a 60-days stay permit stamp
***you now want to do a “border run” and attempt a second visa-exempt entry very close to the end of your recent visa-exempt stay permit
*** in the eyes of some officers of the border Immigration, you are not behaving like a “real tourist”. You are using a 90-days visa plus TWO 60-days visa-exempt entries for a longer stay inside Thailand
*** what the check-in clerk was trying to tell you is, when you return with this flight and enter visa-exempt, you might get pulled aside and asked about your real travel plans and your intentions
In this case you are prepared for the questioning, if you have the three famous proofs:
*** a flight ticket out of Thailand within the 60 days you will get stamped in (which you already have - very good!)
*** a proof of a booked accommodation for a few nights after returning
*** proof of cash. You will have to SHOW a minimum of 20,000 THB or its equivalent in another currency. Credit cards are NOT accepted as a proof of cash, beware!
You DON’T have to pay anything to Immigration. You just need to show with 20,000 THB in cash, that you can finance your next stay