There absolutely needs to be an in-store pickup ONLY option available for products that are ONLY available for pickup. This option needs to replace any other option available under the shipping tab in checkout. The downsides to the solution suggested above are too big to even bother.
The workaround is currently to select "service" when creating a product ... which means, that if I ever want to switch to a shipping option, I have to recreate the bloody product.
Need some flexibility here Squarespace. Pickup is now the standard thanks to Covid, but we need to be able to adapt when needed.