If you’re upselling premium services, donations, or digital products you don’t need to fulfill them the same way as a physical product.
There are two methods to automatically fulfill the upsell product:
1/ Use an app like Auto Fulfill and select the upsell product
2/ Make the upsell product a gift card product. (Shopify guide)
Update the fulfillment settings in your Shopify admin to automatically fulfill gift card products.

You can find this in Shopify Admin > Settings > Checkout and accounts > Scroll down to Order processing.
3/ Use this script to auto fulfill certain products
This will avoid you having orders that are partially fulfilled.