Has anyone created a Per Item shipping module that would only be available as an option after a certain number of items has been purchased?

Example:

I use the table rate to charge shipping on a per item basis.

IF QTY OF TUBERS IS 1-10 SHIPPING IS $7.00
IF QTY OF TUBERS IS 11-15 SHIPPING IS $9.00
IF QTY OF TUBERS IS 16-20 SHIPPING IS $11.00

IF QTY OF TUBERS IS OVER 20 THEN SHIPPING IS $11.00 PLUS $0.25
PER TUBER OVER 20

If I could get the per item shipping module to not be available until 21 items were in the cart then this would allow for the charge of the $0.25 for each item over 20......

Any suggestions are greatly appreciated...

Thanks

George