What you pay Shopify on top of that
The Shopify subscription and payment fees you pay directly to Shopify and to the payment provider, not to me.
The current plan prices are at shopify.com/br/precos. For most stores, Shopify's Basic plan is enough. Transaction fees depend on the payment provider you choose and are charged by that provider.
Maintenance: the same plans as for a website
Basic R$ 249 or Complete R$ 490 per month, final price, required for the first 12 months and cancellable monthly after that. For an online store this means: your domain, theme and app updates, a check of the security settings, a monthly export of products and customers, and help when a question comes up. Hosting is on Shopify itself.
Products, prices and inventory you manage in Shopify; that is what the guidance is for. If you want to change something in the store itself, that comes out of the included hour of the Complete plan or the hourly rate of R$ 290. See the maintenance plans.