After 12May08, it went up to 4¢ to $1.17, incrementing 17¢.  13oz is  $3.21.
 After 11May09 it went up  5¢ 
After 17Apr11  the 1st 3 oz were made all the same & only 2¢ less than what 4 oz used 2B
After 22 Jan12 it went up 24¢.   Wow!  No wonder they publicized only the other changes.
After 27Jan13, it went up 12¢,
After 26Jan14, it went up 25¢.  Wow! No wonder they only talk about 1st class stamps.  Incrementing 17¢.  13oz is  $3.77
After 26Jan14, the increment increased from 17¢/oz to 18¢/oz, so 13oz goes from $3.77 to $4.12
After 31May2015, it went up 22¢.  The increment increased from 18¢ to 20¢, so 13oz goes from $4.12 to $4.54

On 17Jan2016, it was announced that the maximum 1st Class Parcel weight limit had been  increased from 13 oz to 16 oz.
    However, the USPS failed to provide the rates for the additional 14,15, and 16 oz weights in their
    17 January 2016 "Notice 123 Price List", and again failed in their 10 April 2016 "Notice 123 Price List".
    This is because the 16 oz limit applies to COMMERCIAL First-Class Package Service, not to retail service. 

On 10Apr2016, the first 3 ozs went down 9¢ ($2.54 to $2.45) and the increment went down 1¢ (20¢ to 19¢).
    So 13oz is now $4.35. 
    Although the change is supposed to be a roll-back by the Postal Regulatory Commission
    of the "exigent surcharge" of 2014, these are prices we have never had before.  The roll-back is arbitrary.
    The starting price is arbitrary, the increment is arbitrary.
    There will be  chosen reasons for the chosen prices, but the choices and the prices are arbitrary.

On 28Aug2016, the first 3 ozs went up 17¢ ($2.45 to $2.62), then up only 2¢ to $2.64 for four oz,
    before resuming the usual 19¢/oz increments.
    Rates looked like this:

1 oz           $2.62
2 ounces   $2.62
3 ounces   $2.62       
4 ounces   $2.64    and add 19 cents each additional ounce
5 ounces   $2.83
6 ounces   $3.02
7 ounces   $3.21
8 ounces   $3.40                                                           
9 ounces   $3.59
10 ounces $3.78
11 ounces $3.97
12 ounces $4.16
13 ounces $4.35
    14,15, and 16 ounces were announced as acceptable on 7Jan2016,
    but this is only for COMMERCIAL First-Class Package Service, not retail service.

On 22Jan2017, the first ounce increased 5¢ to $2.67, but this rate was extended to the first 4 oz.
The increment was reduced from 19¢ to 18¢, so that the 13oz price actually dropped to $4.29.
 This pattern of charging more for small items but remaining competitive for large ones
was consistent across all changes on that date; namely for First Class letter, flats and parcels,
and for Media Mail. 
 
I may have missed a 2017 mid-year price incease  for 1st class package service.
I show $2.67 for the first 4 oz, but PitneyBowes referred to the 21Jan2018 rate of $3.50 for the first 4 oz as a 50-cent increase.

On 21Jan2018, both the first and second set of 4oz weights were given the same price.
1 oz           $3.50
2 ounces   $3.50
3 ounces   $3.50     
4 ounces   $3.50  
- - - - - - - - - - - - - 
5 ounces   $3.75
6 ounces   $3.75
7 ounces   $3.75
8 ounces   $3.75
- - - - - - - - - - - - -           
9 ounces   $4.10  and 35¢ for each oz more
10 ounces $4.45
11 ounces $4.80
12 ounces $5.15
13 ounces $5.50


 
 


Revised 24Jan2018