Unless you've called your bank and changed it, I don't know of any bank that doesn't have a daily transaction limit. Although most I've seen or heard of are closer to the $1200 range. I'm sure other banks have different limits, just speaking from personal experience.
Give your banks 800 number a try or whatever automated number you'd call to check your balance, they might be closed but there's someone somewhere waiting to cancel your debit card 24 hrs a day! For example, AT&T customer service closes a 8pm cst I believe, but if you end up needing the pin number to unlock your phone, you can get someone 24/7. Even though they're bot customer service, I've never had an issue getting them to help me with something else.
Again, the first thing you should do is give apple a call and find out what exactly kicked the transaction back, all this time you've spent, it very well could just be you typed the address in wrong and the billing address didn't match by 1 letter. They can tell you this much at least I'd think. You can always go to bestbuys website and see if you're local best buy has one. Even if the card is getting kicked back through apples website, it should work for a in person pin based transaction which if it's some type of fraud issue a pin based transaction should still work fine though. Again as previously mentioned you can always call or go to your local apple store. What's the worst they can do, tell you no. Hope it works out for you. Oh, more then likely you should be able to call your bank Monday morning, straighten this out and order your ipad and still get the unlimited plan. I'm not 100% on this, but let's say my electric bill is due on Monday June 7th, and I don't go pay it, they're not gong to cut my lights off until June 8th. So, I'd think you should have until June 7 11:59pm.
Not sure on this either, can AT&T activate a ipad sim in store or does it have o be done from the ipad? I'm getting one in the next few weeks for work and if they can activate the. In store then I'll go ahead and pick one up tomorrow to hold onto for a few weeks. Just a few thoughts for you there!