Shipping for Multiple Quantities - Change Service once Quantity exceeds a set amount

[bigtexcomics]

What I want to accomplish: automatically default to a new shipping method/service once a dollar amount or quantity has been exceeded

Once a buyer buys 11 of an item type, for example, I need to switch to a different shipping service.

I can fit 1-10 of the items in flat rate legal envelope, but once 11 is purchased I need to switch to flat rate medium box.

I don't see any settings in shipping policies or combined shipping that would accomplish this.

Any suggestions?

[fbusoni]

@bigtexcomics 

Have you experimented with variations?  Buy 1 to 1-10 items, pay one price, 11 or more, pay a different price.   You can specify in the listing that the higher price is for a different sort of postage.

Last time I used the variation tool, I do not recall there being the sort of dynamic functionality that you reference above, but I did not look carefully.