I tried that, and spoke to them on phone for 20 mins. Their site says that if I buy without a GSTIN, they will reach out to get some business proof -

"Please note that if you’ve selected ‘No’ option to GSTIN, then our Order Processing team will reach out to you via email for other valid business documents (e.g. Income Tax Return copy, business utility bill or other valid business registration document) to complete the purchase post payment confirmation."