[villgax]

How are you surprised? If you have billing sorted for your SaaS, why do you even need Shopify for?

[mjgs]

Generally speaking it is very surprising for a merchant of any product to require that you not use a competing product in order to use their product.

[teen]

plugins, fulfillment, order tracking, the UI. it does way more than billing.

[system2]

Any of those could be arranged with woocommerce / magento / shipstation.

[jeromegv]

If your SAAS wants to integrate with Shopify (which owns a majority of the market for SMBs e-com websites), telling your customers to switch to magento is a no-go.