I trying to only use Flat Rate Shipping for my website:

I would like to know if there is a way I can do this configuration for my shipping whether its by (each item or by weight)

1 - 2 items - Flat rate Envelope
3 - 6 items - Medium Flat Rate Box
7 - 10 items - Large Flat Rate Box


or

.05lbs - 3lbs - Flat rate Envelope
3.5lbs - 70lbs items - Medium Flat Rate Box / Large Flat Rate Box

I would prefer by each item, but if the configuration is easier to do it by weight I would like to know the coding to place in my admin.