Where is their company located? If they are located in the US, ask them to open the link on the private/incognito browser and try to purchase again. If they have to open a new QBO account, make sure to click the Buy button to get the discounted price for up to 3 months. If they purchase from the Trial account, they have to pay the regular rate.

https:// quickbooks.grsm.io/US
https:// quickbooks.grsm.io/us-promo