Ted here from Shopify, thanks for reaching out to us.
Do you know how the customer is checking out or how they have added products to the cart? It sounds as though they might have done so from your Facebook page. If so, customers can only buy 1 product at a time via Facebook as mentioned in this guide.
If the customer has advised that they were not using Facebook; I was just taking a look at your website there and I was able to add multiple items successfully to the cart. Your checkout seems to be working fine too. Could you tell me the name of the product(s) the customer was hoping to purchase so I can try to replicate the error they are seeing? Do you know what device they are using as well? For example, are they using a mobile, tablet or desktop?
Thanks for letting me know! We'll see how the customer gets on when trying to purchase through the normal website so and if there's anything else I can help you with, please let me know.
Thanks for getting back to me! This seems quite strange and more of a once-off depending on the device or browser they were using. It's quite possible there was a cached file or an extension which was blocking certain features on the site. Have you received a report like this before? If not, I would recommend keeping an eye out for it and if it happens again our Support can raise it further with our Tech Support as long as we have an example and can replicate it.
If there’s anything else I can help you with please let me know!