There shouldn't be any fee at all for receiving from bittylicious - there is a buying fee though but however many BTC you buy you should always have that exact full BTC amount sent to your wallet.
As for paying for lb orders, can you set your own fee in coinbase wallet?
I got stung by the new super high fees on a few orders a couple of weeks a go but then I just changed the fee to 'use replace by fee' and then set it to a low sat/byte, this lowers the sending fee drastically. If coinbase doesn't let you do that you can use a wallet like Electrum which is what I find easy.
Be careful how far in advance you buy your BTC too, I'm sure you don't need me to tell you but for anyone new BTC is volatile af!