This past winter was our 8th or 9th winter in Belize. We knew all about the online forms well before our flight down in late December and I had them all filled out for DH & I. While on our flight down the (WestJet) flight attendants handed out the paper forms to enter as well as the ones to leave the country. I told the woman we didnt need one and she got a little terse with me saying I absolutely needed to fill both out, and shoved them at me. I said first the entrance one is now online and I've already done that and got my QR code, and the 2nd form is for leaving the country so should not be filled out now and there are lots of them in the departures area of BZE. She had no idea that the immigration forms were online. A few minutes later the Purser came to apologize to me and also said she had no idea the forms were available online. I showed her my phone with our QR code and gave her the website and she thanked me.
Also be aware that there were (are still?) some fake websites that are charging a fee to fill out their forms. Folks who were Googling the forms were coming across them. The Belize Immigration does not charge a fee to enter the country as a visitor for the first 30 days. If anybody wants to stay longer than 30 days (your arrival day is counted as day 1) then they would need to go to the Immigration office in the town they are staying in with their passport, a printed statement from their bank showing they had sufficient funds to stay in the country, or proof of their monthly income, and a signed lease/rental agreement showing where (must be BTB approved) they are staying. The cost for that is currently $100.US ($200.BZ) for each 30 day extension with no limit to how many times you can renew.
~Diane