See this post:
https://community.shopify.com/topic/517202
You’d need to be able to use scripts so you’d have to have a plus account.
Topic summary
Main issue: Stores can’t restrict USPS Media Mail at checkout to only eligible products. Customers can select the cheaper Media Mail rate for non-media items, and there’s no built-in way to tag items as Media Mail-eligible vs. not.
Current options and limits:
- Shopify Plus: Possible with Scripts (per linked community post), but unavailable to basic plans.
- Workaround: Create a separate Shipping Profile for non-media products and exclude Media Mail from that profile. This hides Media Mail when any non-media item is in cart. Limitation: Mixed carts (media + non-media) can trigger combined shipping charges and inflated costs.
Community feedback:
- Multiple requests for a native feature to include/exclude Media Mail at the product level. Concerns about potential USPS rule abuse when Media Mail can’t be restricted. Small merchants note Plus/app costs are prohibitive.
Emerging development:
- A participant is finishing an app to hide Media Mail when non-eligible items are present, using Product Type to determine eligibility; awaiting Shopify approval. Others asked for release updates.
Status: No definitive built-in solution yet for non-Plus stores. Thread remains open with a partial workaround and a pending third‑party app.