Whether mailmerge, which both Word and Publisher support, would be of benefit really depends on:
(a) how many letters and certificates at a time you are producing; and
(b) how your Excel data are organised.
Mailmerges are designed for mass-production and really don't offer much benefit for the processing of only a few records at a time.

To be useful as a mailmerge data source, your Excel workbook would have to be set up in a table format, with separate columns for each of the elements needed for the mailmerges (e.g. dates, honorifics, given names/initials, surnames, street addresses, localities, zip/postal codes, state, etc.) and with all elements for a given record on the same row.

And, rather than using separate Word and Publisher files, you should consider whether either of those programs can do the job on their own. It might be possible, for example, to save the Publisher file in Word format, so you can incorporate the certificates into the same document that produces the letters. The converse might also be true.

You'll necessarily end up with at least two files, one being the Excel data source but, depending on your requirements and the data structure, you may not need to do any typing in the mailmerge main document.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]