Although it has not been 5 weeks, but I have done some changes about 2 weeks ago in the backend. If you want the long explanation, here it is.
The creationcassel.com domain is on a shared server. Because of that, the server IP is often flagged by various email providers as "spammy". I know it is not me spamming anyone, but others on the server. For that reason, the host changes the server IP about every day, so what is flagged as spam one day, shouldn't be the next day. However, some email providers are much pickier (gmx was a big one) and just about none of the emails coming from the server (so emails from the store, or from me directly) would not go through.
The scrapbookcampus.com domain, however, is on a VPS so it is not flagged as spammy. So, what I did was get all the emails from the creationcassel.com sites to go through the VPS. This required quite a bit of changes in the DNS, SFP, and such.
The result is that now, invoices from the store, or direct emails from me, go directly through the VPS and won't be flagged as spammy so they finally reach the recipients. So that is the extend that I can do to help. Now, everyone uses their own providers/system and they all seem to have various tolerance level for spam (or what they think it spammy). I also suspect that they might be changing their alogrithm once in a while in order to catch the spammers who try to be smarter. Yeah, it is annoying for you. I wish there was a magical button I could push to fix that for everyone!