Yes Electrum is good but remember you're relying on somebody else's blockchain instead of your own. Though I generally trust the Electrum blockchain servers you never know.
Your online store key, you should be using some sort of script to generate receive payment addresses offline and stick those in a db. The payments should go to a cold wallet you can either with a serial cable send a txn or manually enter the signed transactions, but that's just my paranoid security
The beauty of electrum master public key is that it can be used to generate unlimited number of "receive only" bitcoin addresses. Server script could do it for each sale. If someone hacks into server - he can't steal anything, because Electrum's master private key (to send money) never stored in server.
Your online store key, you should be using some sort of script to generate receive payment addresses offline and stick those in a db. The payments should go to a cold wallet you can either with a serial cable send a txn or manually enter the signed transactions, but that's just my paranoid security