The only time you will pay a fee to Orderinbox is when you sell an NFT. Orderinbox charges a 15% fee per sale. So any time you sell an NFT, 15% of the sale price will go to OIX in order to keep the platform running.
Orderinbox doesn't charge creators to create NFTs. The only fee you will pay when you create an NFT is the gas fee when you mint it on the blockchain.
When you create an NFT, you can select to receive royalties from further sales. This means that every time your NFT is resold you will receive a portion of the sale. Royalties can be set anywhere from 0-30%.