There are no hidden fees, only standard online business expenses like hosting fee: $49 per month. Hosting fee is like having a physical product business and you rent a shop space to sell that product. In the online space you pay a hosting fee, this is a place that gives you a place on the internet to sell your products, the store is hosted on Sales Crafter.
Another expense you'll have is running ads, this is important for you to get sales. Running ads gives you traffic, and traffic gives you sales. No set rule when it comes on to how much you spend on ads, as the goal is always to make more than you spend, so to put it short you spend as much as you can to make a profit. Example if you're spending $10 per day and making $20, then it would make sense to start spending $20 to maybe make $40. Because the bigger the budget, the more visibility your offer gets.