The 30" Column Wrap alone will not support a Mailbox. You will need to building out a frame that is 13 7/8" square and that frame would be able to support a mailbox.
Yes, They can be stacked. To wrap one column, it is recommended the cut be made either at the base or at the top so that they stack appropriately. If you are doing more than one 45" column, cut the second wrap at the top. This leaves the top flange to use on the second column as a starting point. Repeat if more than 2 columns.
These Units do not come with Screws or Caps. Caps are sold separately. Screws are not provided as we would not know what kind of substrate you are attaching these to. Instructions should have been in the Box, but they are available on the website www.nextstone.com.
Caps are sold separately.
Your 6 1/4" post would have to be built out to 13 7/8" squared and then the wraps will fit onto that frame perfectly. The Sandstone 18" caps would fit on top of these column wraps and they can be cut in half with a 6 1/4" opening jigged out to allow the post to go through.
30 inches is the tallest however these column wraps are easily stacked. They are only wraps so it is possible to create a mailbox fitting the inside dimensions of the wrap (13 7/8" squared) and wrapping with these.
Yes these columns build out to 16x16 columns and are 30 inches tall.
Yes these are easily stacked or cut to create any desired height.