This is affecting every Snipcart user who is using Cloudflare.
https://community.cloudflare.com/t/cloudflare-pages-get-rid-of-redundat-308-redirect/324582/22
Snipcart is unable to handle trailing slashes on product URLs which causes error on checkout.
Same issue here with Cloudflare.
For those who encountered that issue with Cloudflare, I found a quite easy workaround. I’ve added a slash at the end of product data-item-url attribute. So instead of having data-item-url=/my-super-product I have data-item-url=/my-super-product/. It did the trick in my case and now I can checkout without any error. 
Hopes it works for you too. 