Unless one resides in one of the few states with no sales tax, there is simply no escaping it now, and not just for ordering Macs online. Thank the state of South Dakota.
Buying a non-standard/BTO configuration severely limits the options, to Apple, or a reseller that offers them, such as B&H, Adorama, Expercom, etc. Best Buy doesn't offer them, nor does Amazon, Costco, Microcenter, or Fry's (which is on its deathbed anyway).
Within those constraints, you'll still have to do a little comparison shopping for the specific combo you want, as the prices will vary a bit.
B&H and the like have some pretty consistent discounts, which are repeatedly touted by sites through affiliate deals, or just by going direct.
Apple offers discounts to students, military, government employees (barely), and some companies through EPP programs. Apple employees get a limited number of big item discounts within a certain time period, but have more flexibility offering friends and family a flat 15% discount, if you happen to have such a connection.
Apple's refurbished store also offers ~%15 discount, and has the full variety of configurations, subject to inventory in stock. It's an easy way to get among the best discounts available, open to everyone, without having to jump through any hoops. The product is basically indistinguishable from new retail product, and offers the same warranty coverage, except it will come in a plain white box and a different serial number. Refurbs are a safe and proven way to get discounted Apple product.
In my shopping experience, aside from the refurb store, B&H probably offers the best discounts, but there are a few caveats with buying from them:
1) They do not accept returns of opened computers, so you must be absolutely sure it's what you want.
2) Cast a wary eye toward a countdown timer next to the price. A model I was considering actually went down by another $50 after the "special price" expired (WTH?!). That's not to say it happens all the time, but it can happen.
3) Their sales tax rebate offer involves applying for their credit card, and paying using that line of credit. If the balance isn't paid in full within the specified period, the interest rate is 30%, which is the typical penalty rate that an average credit card uses as a stick to discourage deadbeats. Applying for a line of new line of credit also involves a hard pull, which may affect your credit score. As a student, the possibility of a obtaining a mortgage may be far off in the future, but for everyone else who is looking to secure a large loan, for a house, or a car, it's generally not a good idea to apply for multiple new lines of credits within a short span. I've worked in finance, and opening new credit lines for one time discounts is generally not a good idea, and could end up costing you thousands more. That game can bring benefits, but is not without risk. Something to consider to in order to save about 300 bucks.
Unfortunately, short of an Apple employee being kind and using their big discount on you, 15% is about the best you can do, until the next new model comes out, and these become the old models. But The Man will still get his cut.
