Crap, that’s what I was afraid of. What about a wallet like Exodus? Do those work differently?
As has been explained already, the problem isn't really from the exchange in this situation, although it is pertinent to use a non custodial wallet to store your holdings, but that wouldn't help this situation. What amount you bought the Bitcoins for in separate transactions doesn't matter when you want to sell.
Let's say you own 1
BTC which you bought in separate 0.5
BTC transactions, the first for $500 and the second for $1,500. If you wish to sell $200 worth of bitcoins now (approximately 0.005
BTC), you simply enter that amount and send. There's no benefit from separating the particular UTXO you purchased higher, as you'll still be spending the exact same amount of Bitcoin for that value.
You can also think of it like cash in your wallet, if you have two jobs and one pays you better for less work, maybe $5 and $20 respectively. You own $25 in your wallet at the end of the day. Would you consider which notes you got cheaper when you want to spend them on groceries? Probably not, as it would not change the amount you're spending out of your wallet.