I previewed XLM on Coinbase and they wanted the same $15 to swap it from US dollars to XLM.
hmmm. I might be getting a bit confused with semantics here but when you say "swapped", do you mean you placed a BUY order for USDT (with USD fiat funds?)
"swapping" probably costs much more than an outright BUY order. (fee-wise)
I would try Coinbase Pro. You can also use on desktop. Easy to instantly send funds from Coinbase to Coinbase Pro for experimentation.
again, here are my fees for "buying" XLM with USD (fiat) on Coinbase PRO (on desktop)
I can't understan why $20 was charged for a $1000 buy of USDT, unless you somehow 'swapped' it instead of placed a 'buy' order. lol, confusing.
And also, the fee to send all the XLM to Bitmart was ....ZERO.