Good Morning @SJBB
Thank you for posting your question in the community, and sorry to hear about the accidental deletion of some of your contacts.
A couple of thoughts.
First, yes, you should be able to simply import the contacts via a CSV file if its easy to source it from Shopify.
Second, you could also try re-syncing your Shopify account with Klaviyo to import historical data. This way, you will also restore the contacts previous transactional history with Klaviyo.
The challenge here will be respecting what the contacts previous subscription status was (subscribed, unsubscribed, never subscribed). If the contacts were deleted (vs. suppressed), I believe any unsubscribed contacts would have been removed as well. I would be cautious mailing these contacts for the first time after import because you might see an increase in spam complaints, etc. if a portion of the list had previously unsubscribed.
I hope that helps!
@In the Inbox
Thank you for your reply. We will follow your advice! Truly helpful.