This is Max from the Shopify Team.
This can happen depending on the settings of the customers' email. Sometimes, automated messages like notifications are filtered into spam or junk folders. One option I'd suggest to help with the potential customer confusion is to display a message on the cart, or the order status page encouraging customers to add your shop email to their contacts list (to avoid the spam folder).
For repeat customers, asking them add your email address to their contacts list will prevent further communications from going to spam.
You could also add this information to any FAQs or contact pages, to be thorough! It looks like you have a Help page at the moment, but, I'd suggest looking into these blog posts for some advice on different pages and why they're useful;
If you have any more questions, please let us know! I hope this added info helps.
Pushing this topic, as recently - after my shop's sales increased to a decent amount per day - had to discover that many confirmation and shipping emails (very important as all the sudden many people asked for status!) went straight into spam folders or got blocked...
Trying the DNS thing now, see if that helps...
Note: Each domain is allowed only one SPF record! If you add more than one entry incorrectly, then all your SPF records will be ignored. You need to merge them all into a single record instead.
Example, if your domain has the record:
v=spf1 include:zoho.com ~all
But you want to add:
v=spf1 include:shops.shopify.com ~all
Your final SPF record will look like this:
v=spf1 include:zoho.com include:shops.shopify.com ~all