My name is Rae and I work for Shopify. Thanks for posting this question.
To ensure that customers are only checking out with PayPal as their payment method on your site, please follow these steps to set up PayPal Express Checkout in your admin.
Then, in ‘Settings > Payment Providers’, please ensure that no other payment methods are currently set up on your site. If any other payment option is active, you will be able to click ‘Edit > Deactivate’ on that payment method to disable it.
Once you have PayPal set up and all other gateways disabled, I recommend having a friend or family member place an order on your store and complete the checkout. This will help give you an idea of how the process works, and how it looks on your end when an order comes through. Having someone close to you provide feedback about the experience can be super helpful for improving your customer’s checkout experience overall, so it’s really important to do this.
Please give this a try and let me know if you run into any issues!