US Postage not calculated correctly by weight

[Twister]

Need Help!
We sell very small items and ship by U.S. Mail. I'd really like to adjust shipping cost by number of items using ranges of items(1-6 items = $4.35, 7-10 items = $6.35, 11-14 items = $8.35, etc).
Since all items are about the same weight I set each item's weight to 1.00 and set the table up like this: 6:4.35,10:6.35,14:8.35, etc. but the rates are not not being calculated correctly - they seem to increase randomly and too soon. For instance, 4 items calculate correctly but 5 items move to the next higher rate, and so on.
Table calculations by price work fine but I can't use them - without a mod to calculate shipping by ranges (groups) of items I'll need to use weight.
Any ideas or suggestions?