This add on works perfectly.
The only problem is with the USPS itself.
USPS has a 1st class shipping rate for small and low-weight items, like envelopes. We can ship the catalog for under $1.50 1st class mail and cheaper if we use Parcel Post (for bulkier, lightweight items like bubble envelopes which we sometimes send replacement parts etc).
When people order our catalog, we send it free (no charge).
However, the USPS Click-n-Ship website will not support 1st class mail. The cheapest option is Flat Rate for between $5.15-$5.50 or so depending on the box or envelope etc. So while Click-n-Ship is great, and this add on works perfectly for regular orders, the Post Office might force you to use more postage than you need.
If you send in boxes, the Flat Rate boxes are awesome and we use them a lot too but our #1 mailing is the catalog and this means we need to find another option.

Thanks for the add on we are using it, and it works great. We just can't use it for all the orders we get.