it's normal to pay a high fee like 31$ when you send 50$? I know its my mistake that I didn't check before I sent
No it's not, at least not most of the times.
You can always check the state of the mempool and see how much you have to pay on average at the moment of transaction.
This all applies if you are using non-custodial wallets that have the option for manually changing the fees.
If you are using centralized exchange or custodial wallet this rule does not apply, they can charge any fee they want.