VERIFIED SOLUTION i

MailStream Plus- Standard Mail- Why might a smaller Carrier Route not get the High-Density (HD) or High-Density Plus (HDP) rates?


 
The USPS's DMM (Domestic Mail Manual) has the minimums required for qualifying for Standard Mail Carrier Route (CRRT) HD and HDP levels.  Section 243 6.4.2 says for Letters-
------------------------------------
Basic Eligibility Standards for High Density and High Density Plus Prices for Letters
All pieces mailed at high density or high density plus prices must be prepared in walk sequence according to schemes prescribed by the USPS (see 245.9.8 through 245.9.9). Multiple pieces per delivery address can count toward both density standards. Specific density requirements are as follows:

a. Pieces mailed at high density prices must be sorted together in sequence in quantities of at least 125 pieces for each carrier route.
b. Pieces mailed at high density plus prices must be sorted together in sequence in quantities of at least 300 pieces for each carrier route
.
------------------------------------

245 6.5b gives the same minimums for flats-
------------------------------------
Meet the density requirement of at least 125 pieces for each carrier route. For high density plus prices, the density requirement is at least 300 pieces for each carrier route. Multiple pieces per delivery address can count toward the density standards, except for pieces with simplified addresses as allowed under 602.3.0.
------------------------------------

So you have to have a minimum 125 pcs to the CRRT for HD, and 300 pcs for HDP.   Mail to a smaller CRRT may not qualify.

(The DMM references above are as-of July 2015.)
UPDATED:  April 19, 2017