Trademark registrations are a formality in the US, but saves everyone a lot of time and money. In the US, it goes to whoever uses it first in commercial capacity. They will have to dispute it in court because they were using "Everyday Sling" in practice way before Amazon was.
The better argument for Amazon is that "Everyday Sling" is too generic of language similar to "Daypacks," and even Peak Design may have thought so because they did not bother to register it for years [0], [1].
> The better argument for Amazon is that "Everyday Sling" is too generic of language similar to "Daypacks,"
I don't really agree with that, because the "everyday sling" is not a sling, it's a container. It would appear to be a fanny pack supported by your shoulder instead of your waist.
A sling is a piece of cloth which supports something against gravity; the major applications are holding an injured arm and holding a baby. See https://www.merriam-webster.com/dictionary/sling ; the ordinary sense is noun 2(a), and there is no sense that includes a container.
[1] - https://uspto.report/TM/90489397
[2] - https://camelcamelcamel.com/product/B084CG43XD