Shipping with percent and flat rates

[spamis]

I am new to oscommerce so please be patient. We sell products in bulk and ship by price like this:

 

0.00-5.00/3.00 shipping

5.01-26.25/7.00

26.26-100.00/10.00

>100.00/8%

 

I can get the shipping table to work with the flat rates but the 8% rate above $100.00 doesn't work. Merely entering the 8% creates a shipping cost of $0.00 (a bargain). I have found a few posts ranging from entering the rates as above to adding a new module entitled "percent.php." I have tried both and neither work. From other posts it seems like I should be able to do this. Advise please if it can be done and if so, how to do it. Thanks.