You could do that. If your customer list is huge and/or they're downloading
the data a lot, a different data sharing mechanism might be better (e.g.
FTPing CSV files that they pick up daily, etc.). You could also just send
them updates (i.e. list of people subscribed/unsubscribed since XX date)
instead of your entire list.

If the data size is still too big, it might be better to instead publish a
web service method like CheckUserRegistration() where they pass a user's
information & you return whether they're in your system or not. It has the
advantage of being a real-time check (instead of a daily download that might
get out of date as the day progresses), and it might also be faster than
huge, repeated downloads. You could also expose a web service method
allowing them to POST registration information, instead of sending you
Excel.

When they send you Excel, you could use the Excel COM object or ADO.NET to
load the data programmatically into your system. I am assuming that the
labor intensive aspect often comes w/ bad data, not necessarily importing
good data.

In terms of securing web services, SSL is good (maybe w/ custom certificates
that you generate), but has a performance impact. You could also look at WSE
(http://msdn.microsoft.com/webservices/building/wse/default.aspx) in terms
of web service security.

HTH,