Airtable is free to sign up and free for their basic account which enables you to have upto 1200 records and 2GB of file attachments. 


Visit the Airtable Pricing page for more information and to sign up for an account. 


Typically a free account should be sufficient for people upto Merch by Amazon Tier 500 or 1000. 


The Plus account gives you upto 5,000 records and 5GB of file attachments, which should be enough for upto Tiers 2000/4000/6000


If these aren't enough for your flourishing Print on Demand business, then they have an all singing all dancing Pro Tier for upto 50,000 records and 20GB of attachments. 


Above this and you can contact them for a custom solution to suit your needs. Airtable Pricing