Can nouz change anything in my store?

No. Access is read-only. nouz can read orders, refunds, products and payment transactions and nothing else; there is no path in the product that writes to your shop, and the scopes you approve on Shopify's own screen say exactly that.

It also never places a script on your storefront, so your page speed, your checkout and your customers' browsers are untouched. Costs and weights you enter in nouz live in nouz.