The out of the box shipping rate provider " Manual (Fixed or By Weight and By Total)" has fields for "Order subtotal from" and "Order subtotal to".
However, it won't handle "All gift vouchers regardless of value if that is the only thing ordered ..." .
(And, I assume you also don't want to include the price of the gift vouchers in the from/to range above, when there are other items in the cart). It will require customization (make your own plugin; you can copy that one as a start), or
Shipping Director can do it - example: You can still set up your ranges in FixedByWeightByTotal and SD can call it. SD will "pack" items excluding gift vouchers:
OptionExit
Shipping
Items.All(HasCategory("Gift Vouchers"))
2.99
Packing
Pack exclude gift vouchers
true
Packing.FirstFitSingleBox
HasCategory("Gift Vouchers") [this is the Exclusion Expression]
Option
Shipping
true
Shipping.FixedByWeightByTotal