I'm still having a hard time viewing it as proof of a religious bias. There's still a number of conclusions that could be drawn that aren't that... The decorative tape might be harder for automated postage-processing machines to deal with. The decorative tape might be less sticky than the non-decorative tape (they make no mention of that). I'm not sure how they delivered this stuff to the Post Office, but if the packages were presorted into a group of decorative tape boxes and a group of plain tape boxes, that could be the _entire_ reason one type was lost/delayed more than the other (if a whole bin or a pallet or whatever had an issue).
It's only 9 packages that went missing, too. So if 5 of those 9 went to one city, 3 of the 9 to another, and 1 of the 9 to a third city, then the conclusion to draw is that those 3 cities had issue with the decorative tape for some reason. Not that the entire USPS is religiously biased. If each loss were stated to be in a separate (and significantly different) city, that would be closer to the stated conclusion.
Finally, at the bottom in the small print (I read it a bit more carefully this time), you see that they were shipped via Deutsch Post then handed over to USPS when they reached the US. (That's pretty standard for how international shipping works.) Without tracking, how can they lay blame for lost/delayed packages solely on the USPS? And even if they had tracking only up until the packages left the country (as is how some international shipments work), there are still factors outside of the USPS that could affect this, for instance maybe someone at the Deutsche Post decided for fun (or maybe even some sort of processing reason) to sort all/some of the 178 packages. Then imagine that a single bin of packages went missing and it just happened to be one of the bins of decoratively-taped packages... That could mean one act of incompetence (or even maliciousness) is deciding the entire outcome of the study.
Really though, the fact that they're concluding this based on 10 lost packages still strikes me as odd. An issue with just 10 packages is so well below the levels of extreme incompetence that I've encountered with the Post Office in my years that claiming religious bias feels like giving the system far too much credit.