We operate a mail order company that has roughly 20-25 thousand items
in its catalog.  We use an inventory management software program
called Mail Order Manager.  The company that produces the software,
Dydacomp, offers a service
called SiteLink that takes our data and uses it to create a web site
where customers can browse, search, and purchase from our catalog. 
Our data is stored on the SiteLink server and we send in small updates
once or twice a day.  We do not process orders in real time online;
orders generated from our site are downloaded manually by us and
processed at that time.

For a wide variety of reasons, we have become exceedingly unhappy with
SiteLink and are now desperate enough to consider transitioning to a
different program.  We are not particularly happy with Mail Order
Manager but do not have the budget to also change inventory management
software.

The question is: given our desire to switch away from the SiteLink
service and our unfortunate budgetary constraints that obligate us to
stay with Mail Order Manager, what program or system can we use that
will be compatible with our existing data (which is constantly updated
on a daily basis)?  The new system needs to serve essentially the same
function as SiteLink (take our inventory data and use it to create a
website that is capable of taking orders).  It also needs to be
capable of handling a large catalog.

If by some miracle there is extremely inexpensive or free software
which would replace -both- SiteLink and Mail Order Manager (and we
could transition our existing data), that would also be an acceptable
answer.