Hey David49, you may want to check out Dynamic Checkout Buttons. Your theme may already include them. If so, you can turn them on and add them to product pages.
Quoting the Shopify docs
- From your Shopify admin, go to Online Store > Themes.
Find the theme that you want to edit and click Customize.
From the top bar drop-down menu, select Product pages.
In the left-hand toolbar, click Product pages.
Check or uncheck Show dynamic checkout button.
Hope that helps!