It's the size of the beasts that matters more than the weight.
If they fit into either a

13-5/8" x 11-7/8" x 3-3/8"
or
11" x 8-1/2" x 5-1/2"

Medium Flat Rate box, then Postage at the Post Office is $12.65 or $11.30 if the postage is bought online.

If you have to go to the Large Box then the postage is in the $18 range.

When I checked online, they did not even list a "parcel" rate for a 7 lb. package.