Why does Merchant Center say my Shopify product has a mismatched price?

Short answer

Google compares the price your store sent with the price it reads on the product page. When they differ, the product can be disapproved, and Google says it may lead to an account suspension. The usual causes are a recent price change that has not reached Merchant Center yet, wrong price data in the page's structured data, or a page that shows Google's crawler a different price or currency than it shows you.

What Google compares

Merchant Center holds the price your store sent. Google also visits the product page and reads the price there, including from the page's structured data. The issue appears when the two are different. Google says this may lead to an account suspension, so it is worth fixing even when only a few products are flagged.

The causes Google lists

A delay between a price change on your site and the update of your product data.
Structured data on the page that gives Google the wrong price.
A page that shows crawlers a different price or currency than it shows shoppers.
Sale price dates or time zones that do not line up.
Products sold in a minimum quantity. Google wants the total price for that quantity.

Google also notes that a mismatch it found earlier may already have been corrected and reapproved by the time you look.

Where to look on Shopify

Think about when the price last changed. If it was recent, the mismatch may be the gap between your change and the next update of your product data. Google lists this as the first cause.
Run the product page through Google's Rich Results Test. It shows the price and currency Google reads from the page. If that is not the price a shopper sees, the theme or an app is writing the wrong value.
Look at the page as a shopper in the target country would. Shopify says a shopper who clicks a Google ad lands on a page that shows the currency and price of the ad. If your store changes currency or price by location, compare what that country sees with what Merchant Center holds.
If you use Markets with custom catalog pricing, look for Shopify's warning. Shopify shows one when you apply custom pricing to a market for a channel like Google, because shoppers expect the price they saw.
If the product is sold in a minimum quantity, submit the total price for that quantity.

Getting it checked again

Fix the cause first. Then in Merchant Center open Products, go to the Needs attention tab and request a review. Google says a website review can take up to 12 hours, and you cannot send another request while one is running.

If automatic item updates are on

With automatic item updates on, Google changes the price in Merchant Center to match your page and reports it as "Automatic updates: Mismatched price". The product data your store sent was still wrong at that moment, so treat it as the same problem and find the cause.

Questions

Is this a warning or a disapproval?

Google says products may be disapproved when the prices differ, and that mismatched prices may lead to an account suspension.