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Can I use ETH from metamask to Keystore arb ETH?

If your Metamask ETH is on Arbitrum, otherwise you have to bridge it and if it's on mainnet, then it's gonna be expensive

Yeah, it's on mainnet. I would rather buy ETH on arbitrum.

Then it might be expensive to bridge that to Arb. I would bridge from another L2, such as Polygon, OP, or BSC.
Been using deBridge: https://app.debridge.finance/r/8873
It's a great one!