Zoho Inventory, is free until 50 orders per month and generate invoices, hat a direct free connector to Shopify and 1000 oder features. Zoho Invoice is 100% free, no limits, you can integrate it with Shopify using a tool like Make.com where the first...

You can use base.com (I am not related)… it is an amazing platform, beside connecting Shopify to other marketplaces, accounting programs, carriers and so on, it also creates invoices. It is free until 100 orders per month. 

https://apps.shopify.com/dynamic-variant-description