No Shipping Available Problem

Please i need little help!
Whenever customer place an order with 20 quantity or more pcs ,he face following message.
no shipping options available...

but it's fine when he place with 5 and when try to exceed from 5 he get no shipping available from..please see attachments when he go through 5 quantity it's showing follwing and how i can fix it please.
we need an option that the envelope option goes away after person buys more than 3 pcs of any combined battery or adapter
and then the regular rates come up
and the rates need to include dimensions
for example 30 pcs may fit in a big box like 16x14x10 as an example
that is in inches
and it would be at least 30 lbs

our shipping charges have ALWAYS been inaccurate because of this
customer always has paid less