Store Checkout with Custom Price

If it is your own custom created store then you can just use the html component and send data from the page to it and from it back to the page, or just one of those options.
https://support.wix.com/en/article/corvid-working-with-the-html-element
https://www.wix.com/corvid/reference/$w.HtmlComponent.html

If you are using Wix Stores itself, then you can either read their own support pages or contact Wix Support themselves. https://support.wix.com/en/about-wix/contacting-wix-support

https://support.wix.com/en/wix-stores/setting-up-your-store
https://support.wix.com/en/article/request-adding-a-form-to-the-cart-or-checkout-page-in-wix-stores
https://support.wix.com/en/article/request-customizing-the-checkout-page-in-the-wix-stores
https://support.wix.com/en/article/request-embedding-external-html-code-to-a-product-page-in-wix-stores
https://support.wix.com/en/article/request-allowing-customers-to-set-prices-in-wix-stores

Wix Stores does have a product configurator option that might be of use to you if you use Wix Stores.
https://support.wix.com/en/article/corvid-tutorial-adding-a-product-configurator-to-a-wix-stores-site