I think OP is talking about Tron token itself and not Tron USDT or token built on Tron network. Transferring Tron token itself in most exchange is cheap as it cost just few cent.
ETH and Tron fee are nothing different, actually Bitcoin fee is way more cheaper even though it's decentralized and has huge volume.
Bitcoin should have been the best for any organization but since OP is trying to talk about cheap transaction here, I think BNB may be a better option.
OP was talking about the blockchain that is used by Paypal for their Stable Coin, not the cryptocurrency they used. Paypal can't use Bitcoin blockchain to deploy their stable Coin.
I don't think Paypal is wrong for using Ethereum, compared to Ethereum, BSC and Tron is relatively new, and less reputable, as a company that already big and has reputation PayPal decision to choose Ethereum is understandable. Tho adding other blockchain support for their stable coin would attract more people indeed.