Celia is designed so you always know the cost before you commit. Whenever you buy, sell, or withdraw, the app shows you the fee and the exchange rate up front, with clear "You receive" and "Fee charged" lines, so you can see exactly what you will pay and get before you confirm.
A fee may be a small percentage with a minimum, and the buy or sell rate already includes Celia's rate, all of it is baked into the quote you approve. If an amount is too small to cover the fee, the app will tell you (for example "Amount too small"), and you simply enter a larger amount.
One thing to keep separate: on-chain gas fees are not the same as Celia's fees. Gas is charged by the blockchain and paid in the network's own coin (see the gas article).