Actually, the onus is on the CA only to show it was sent successfully. What happens after that is not really their problem. If the debtor is going to claim it was received significantly later than the CA can show it was sent, I believe the onus would be on him or her to establish some sort of fraud or conspiracy.
No need to go to the level of "fraud" or "conspiracy" to rebut the presumption of the applicability of the "common-law mailbox rule". All the rule does is set one date certain: The date that the letter was turned over to the USPS for processing and delivery (the only date the CA has any control over).
What would happen? All the OP would need to do is establish that they did not have the letter in hand
before they date they claim. Yes, the letter was delivered no later than the day they returned to "home base", but when did it actually get there? The date it was delivered, absent certified delivery proof of mailing, cannot be established with any degree of certainty.
The "mailing time" usually allowed for in case law is also a presumption: The recipient was there to get the letter they certainly were not expecting! The OP in this case, however, could easily rebut this presumption since they were not available to receive the letter before they did due to no fault of their own.
The fact that the letter was promptly replied to would also help here to show that the effective delivery date was the day they got back and the first possible day that they could have known about the letter.
[The case most often cited as precedent for the "common-law mailbox rule"--"Mahon v. Credit Bureau of Placer County"
--involved a situation in which there was NO question that the mailing took place following acceptable procedures AND that the Mahons were available to receive the letter the moment it dropped in the mailbox.