In the table published at this U.S. Postal Service Website, USPS compares different service classes in different aspects. One aspect is called "incentive". I wonder what it means? From Google, I guess it may mean "A payment or concession to stimulate greater output or investment", but this meaning doesn't explain the aspect in the USPS table well to me.
Generically, the USPS defines "incentive programs" to mean:
Incentive Programs allow customers to participate in marketing campaigns offered by the USPS®. Customers can register for these events and qualify for incentives available for certain products offered by USPS. Conditions and terms will vary.
That said, after looking through the links provided in JLG's comments, browsing some USPS websites, and talking to my cousin last night (who works for the USPS), I can't find any incentive program other than the one that gives a 2% discount if registered business customers use barcodes on their mailings. (Intuitively, such a discount would make sense, as the barcodes would reduce USPS labor costs by allowing the envelopes to be sorted via automation, rather than by hand.)
However, a USPS website says:
The 2012 Mobile Commerce and Personalization Promotion will provide business mailers with an upfront 2% postage discount on Standard Mail® and First-Class Mail® letters, flats, and cards (presort and automation only) that include a two-dimensional (2-D) barcode or print/mobile technology that can be read or scanned by a mobile device.
Similar to last year's Mobile Barcode promotion; this promotion focuses on the integration of mobile technologies with direct mail as a means of reinforcing mail's continuing relevance as a marketing channel. The 2012 promotion specifies that barcode or similar technology must lead the recipient directly to either: (1) a webpage that allows the recipient to purchase a product or service on a mobile device, or (2) a personalized URL, which leads to a web page that is unique to an individual recipient. Best practices such as directional copy and mobile optimization are also required.
Stamps.com mentions up to four ways an envelope can be barcoded:
but I'm not sure if any of those barcodes would lead someone to "a webpage that allows the recipient to purchase a product or service on a mobile device, or a personalized URL, which leads to a web page that is unique to an individual recipient," causing me to wonder if any of those barcode(s) would qualify a registered business customer for the discount.
I don't believe any volume discount applies when referring to "incentives" in this context. The USPS does offer bulk mailing rates, but I don't think that falls under "incentives" in the table at the O.P.'s website.
Moreover, incentive programs don't seem to be run year-round; the one mentioned above ends at the end of August 2012.