+ AA6YQ comments below
If you hold the Ctrl key while clicking on Sync eQSL.cc QSLs, that
will download *all* QSLs in your account/nickname.
I regularly download QSLs on my office laptop (my "QSL" system)
and then archive any QSLs. One it a while (every week or two)
I sync eQSL on my shack computer - the shack computer gets QSLs
that have been archived because DXKeeper asks for all QSLs "since"
the last download date.
+ If you point me at the eQSL.cc documentation stating that this procedure will report archived QSLs, I will update the DXKeeper documentation accordingly. Thanks!