Is it possible to calculate potential UPS tracking # using algorithm?

[Voixdelion]

Geeky question here -

Recently bought something on ebay and the seller has given me a tracking # that appears to be missing a couple (2) digits, but I cannot get hold of them to correct the problem.  I was wondering if it would be somewhat possible to narrow down which digits are missing by using some algorithm to calculate it in reverse since I know some of the required info and according to this site : ""1Z" tracking numbers are of the form 1Z xxx xxx yy zzzz zzz c, where xxx xxx is the alphanumeric account number of the shipper, yy is the service code, zzzz zzz is the package identifier, and c is a checksum of some sort"

So the number I have is 1ZRW560329877330 and it appears that based on comparing most of my UPS tracking numbers it looks like the two missing digits are part of the XXX XXX portion and I know that it is shipping from a ups office in johnson, TN -signature required and insured for $150 - I believe it went ground, but possibly 3 day select.  

Will any of this help me figure out the likely missing digits?  Lining it up next to other tracking numbers the one consistent thing seems to be the "03"  in the middle which means that my number probably looks like the following with an unknown positions between the question marks:
1Z  ?RW56?  03 2987 733 0

There are also some key bits of deciphering info presented here on this site  but I have no idea if it  is useful

Is this a solvable problem via mathematics?  Or are there any other positional indicators that might hint at the possible missing digits?  whaddaya think?

[asdf]

I've researched this subject once before.

  1. The first two characters must be "1Z".
   2. The next 6 characters we fill with our UPS account number "XXXXXX".
   3. The next 2 characters denote the service type:
          * "01" for Next Day Air shipments.
          * "02" for Second Day Air shipments.
          * "03" for Ground shipments.
   4. The next 5 characters is our invoice number (our invoices are 6 digits; we drop the first digit, e.g., the 123456 invoice would yield 23456 characters).
   5. The next 2 digits is the package number, zero filled. E.g., package 1 is "01", 2 is "02".
   6. The last and final character is the check digit.

source

[Voixdelion]

so assuming that the guy sent it via a retail ups outlet, then would that office have a potentially discoverable account number? (with 4 of the six digits being known?)