What are best mailers for financially sensitive docs?

[Saod85]

Greetings. I'm hoping this is the right place to ask my
question.

I work in the mailings division for a financial services
company. Our mail production system right now is outdated
and it is becoming increasingly difficult to handle
customer communications, especially when checks or other
financially sensitive documents are involved. We have been
losing too many pieces, and this is adding a lot to our
overall costs because there's often phone service time
resulting from these irregularities.

I'm in the early stages of researching a new system to
implement that can handle all of our mail components, from
checks to marketing inserts, and hopefully reduce our costs
in the long run. As far as the industry big boys are
concerned, are there systems that are preferred for more
sensitive financial documents with minimal loss of inserts?
Are the error rates vendors supply accurate?

Thnxia for any advice/suggestions!