Since you only want to minimize costs, send from Blockchain directly to the buyer's address and choose the cheapest fee advertised on https://mempool.space/ if possible, or choose the slow transaction (cheaper) if you want to spend them. If you want to keep them, create a wallet in Electrum; by default, they are SegWit (bc1), and since SegWit transactions are smaller and more efficient, they are cheaper in terms of fees.

Or, as mentioned above, export the private key into Electrum and customize your fees(Verify that it is not lower than the recommended one to avoid getting stuck in the mempool).