You say you sent one zipped and the other not, but you don't mention password protecting the zipped file, so there is every likelihood that neither got to avast.com, just as your Yahoo mail experience shows. Email servers on route are likely to have anti-virus scans which can open zipped files to scan, applying a password stops that.
You could also add it to the avast chest, User Files section (File, Add) and send it from there, it will be sent encrypted, submissions sent from the chest are detected upon receipt and filtered from those coming in in the way you sent yours. This should help as over 4000 email are received at the virus@avast.com address.