Hey @Ce_Cui,
Similar question here: Clarification on Billing Requirements — App Using Stripe for Subscription & One-Time Charges - #2 by KyleG-Shopify
If Shopify Billing is absolutely not going to work and you need to seek an exemption, the path for that is to include your request and the reasoning on why it’s needed in your app review notes. Your app reviewer will review that during the review to determine if an exemption can be applied.
To set expectations though, exemptions are rare, and your scenario above would likely not be granted an exemption. It is expected for there to be a path for new merchants to install your app and be able to pay through Shopify billing but, you don’t need to change anything for your current clients.