Entering input causes modal reloading automatically
It seems like you're declaring your hooks outside your component. Try putting them inside your ReturnModal function instead, like this:
const ReturnModal = () => {
const [price, setPrice] = useState(0);
const [shippingCost, setShippingCost] = useState(0);
...
Documentation reference: Using the State Hook.
Also, I would strongly recommend using the React Hooks ESLint Plugin (among others) to detect issues with your hooks. Here is a guide on how to add this to your React Native project: Add Eslint Support to your React Native Project + React Hooks Rules.