All qualifying items will have the following displayed on the page:
Buy 2, get 20% off
Until November 13th, when you purchase two select DEWALT power tool accessories get 20% off, when you purchase three get 30% off when shipped and sold by Amazon. Exclusions apply. See discount when items are placed in cart.
You can always check to see if it's qualifying by adding multiples of the item and see if you are getting the 20% off discount. For some odd reason, the 30% does not seem to work and apply. I contacted amazon customer service and they do not know why either as all the items that qualify for 20% off should also qualify for 30% off. Basically, they can do manual adjustments for the difference in price. They can refund you the difference when the item ships, or immediately refund the difference in the form of their amazon store credit (which is even easier for a lot of people so they don't have the hassle of checking their credit cards or contacting again for the refund in case anything goes wrong. but usually, Amazon is pretty good and refund gets processed automatically).
Here are a few notable items that have been on SD in the past and with the promotion, may be cheaper especially with the 30% off.
With 30% off:
Medium Tough Case $4.19
Reciprocating Saw Blades (6) $5.59
29 Piece Screwdriving & Nutdriving Set $7.48 -- now $9.79
14 Piece Titanium Drill Bit Set $9.09
Bi Metal Hole Saw Kit $31.49
Of course there are lots more...