Additional shipping option

[pest3366]

Right now the only option for shipping I have for people who spend more $ is to change it on every individual invoice.
I was thinking that in defaults I could mark a box and add a $ amount and if a customer's cart is that amount (or more) than I could set an amount for shipping ie $5 or $10 or free or maybe a % like 50% off s/h.
Similar to the additional item spot. This way I could use both. Add per item but if the total is enough I wouldn't charge anything.
Example :
buys 1 item $5 s/h $3.50
Adds another item $2.50 additional $1
adds another item $10.00 additional $1
adds another item $25.00 additional $1
Total $42.50 + $6.50 s/h But in this case I would want to give 50% off s/h. Or maybe even change to free shipping.