VERIFIED SOLUTION i

What is the minimum number of letter trays (or weight in bundles or sacked mail) required to make a pallet. MailStream Plus

Product affected: MailStream Plus™, all versions, all platforms
The minimum normally is 36 ft of trays (18 2-foot trays or equivalent in mixture of 1 and/or 2 foot trays), unless you are entering the mail at the actual facility.  For sacks it's normally 250 pounds unless entering at the facility..

DMM 705 (as of May 2019) says-
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 8.5.3 Minimum Load
The following minimum load standards apply to mail prepared on pallets:

a. For Periodicals, USPS Marketing Mail, Parcel Select, and Package Services (except for Parcel Select mailed at DSCF and DDU prices):
  1. In a single mailing, the minimum load per pallet is 250 pounds of bundles, parcels, or sacks, except as provided in items 2 through 4 below. When preparing letter trays on pallets, the minimum load is 36 linear feet or three layers of trays, except as provided in items 2 and 4 below.
  2. There is no minimum load for pallets entered at a destination delivery unit (DDU) if the mail on those pallets is for that unit‘s service area.
  3. The minimum load for pallets is 200 pounds of USPS Marketing Mail parcels (machinable or irregulars) entered at origin (NDC only) or at a DNDC to claim the 5-digit or NDC price.
  4. A pallet may contain a minimum of 100 pounds of nonletter-size mail or 12 linear feet of letter trays if it is a NDC or ASF pallet entered at the destination NDC or ASF; an ADC pallet entered at the destination ADC; an SCF pallet entered at the destination SCF; or the only pallet entered at an individual destination NDC or ASF, ADC, or SCF facility.
  5. At an SCF, a 5-digit, 3-digit, or SCF pallet may contain less than the required minimum load of mail for that SCF’s service area only if the SCF manager provides written authorization for such preparation.

b. Parcel Select mailed at DSCF and DDU prices. A minimum load for the price claimed may be stated in terms of weight, combined piece minimum and weight, or minimum height. Mail entered at the Parcel Select DSCF prices and prepared directly on pallets or in pallet boxes on pallets must be prepared under either a minimum height requirement or under one of two options requiring a minimum number of pieces and pounds per pallet. There is no minimum weight requirement for an SCF pallet containing 5-digit scheme and 5-digit sacks prepared for the DSCF price. There are no minimums for the Parcel Select DDU price.

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a.1. above says that you normally need at least 36 linear feet of trays (3-tiers) to make a pallet unless you are entering the mail at the actual destination facility. Or normally 250 pounds of bundles or sacks. 
It also clarifies that pallets of bundles, parcels, or sacks are prepared based on weight, and letter trays on pallets are based on Linear Feet (trays) 
UPDATED:  May 16, 2019