if you see things like usdt/eth, usdc/eth, usdc/sol, etc. The first part is the crypto (coin) you pay in, the second is the network you send it on. The third part, is that you have to have some of the network native coin to pay the network fees to send the payment (its called gas).
so, if a vendor requires payment in usdt/eth, you need to pay the requested amount in USDT, send on the Ethereum (ERC-20) network. You also need to have a small amount of ETH in your wallet to pay the gas to transfer the payment.
You can lose all of your sent payment if you don't use the right network, primarily with ETH as there is the ERC20 network, and the BASE network. I have yet to see a vendor request payment via the base network.
Read all the crypto information you can find before sending anything.
SEND A VERY SMALL TEST TRANSFER AND ASK THE SUPPLIER TO VERIFY RECEIPT OF THE TEST BEFORE MAKING THE FULL PAYMENT IF YOU HAVEN'T TRANSFERED BEFORE.
You'd rather lose $5 than $500