The biggest limitation to your email merging is the inability to attach other
documents. This in many occasions has forced us to use another product, which
allows this type of feature.

The most intersted fact of all is that you are aware of this problem, as it
appears on your website, see below:

You can't include an attachment
It's perfectly natural to want to include an attachment in an e-mail message
that you send by using mail merge. For example, you might want to attach a
document that lists all of your coffee roasts. Unfortunately, the mail-merge
process doesn't support attachments.