Fear not!  Bitmain is the best shipping from China I have ever heard of.  There is no extra tax or shipping when ordering from Bitmain directly.  My 3rd and 4th S3's from batch 5 will be arriving here in FL tomorrow or Thursday.  My 1st and 2nd were from batch 1.  The only significant difference between batches (that I have heard of so far) is price and shipping date.  Although some are reporting that batch 4+ miners are hashing faster than B1-3.

I don't really know anything about CoinBase, but I can't imagine why you couldn't pay directly from their site.  Transaction donations these days should be between .001 and .005 BTC for reasonable processing time.

That PSU looks good as long as it's one S3 per PSU .  Don't try to stack two to one.  But with 4PCI-e cables, it should be able to overclock as much as your S3's can handle.

Coinbase pays the miners fee themselves, so you don't need to put in any extra.
However, it takes for coinbase up to 1hr sometimes to have the first confirmation, so your transaction would timed out initially, but they (bitmain) usually reconcile it later either by themselves or if you send them email with your transaction id. If time is of no particular importance, but you want a peace of mind, send your coinbase coin to blockchain.info wallet first and pay from there, paying minimal fee of 0.0001 btc or slightlly above.