Should I sign in to email in Merca?

Yes.

The mailbox that receives this store's platform mail should be signed in — and opened — in that store's window. Don't use the browser you use every day, and don't tap links from those messages on your phone. How to save the mailbox is in How to add an additional (email) account.

Platforms such as Amazon send messages with links, for example to confirm an account, handle an order, or open the seller pages. If you tap one outside Merca, the page uses your everyday browser and this computer's network, not this store's node and device fingerprint. Mixing the two can put the store at risk. If a message has a link, open it in this store's window.

Some messages report back to the sender as soon as you open or preview them: where they were opened, and on which network. Opened in your everyday mailbox or on your phone, that report is this computer's or phone's real network. Opened in this store's window, the sender sees this store's node. See How unique device fingerprints work. How traffic leaves is in How the proxy chain works.