You're vastly underestimating what sort of effects "peak season" has. UPS hires 95,000 temporary employees each year; FedEx hires 50,000. Amazon hires 120,000. Retail scales up similarly, but I'm less familiar with the industry.
"A few thousand" is how much single distribution centers scale up employment.
So Amazon currently hires 120,000 people at $12/hr for 10 weeks for a total cost of somewhere near $576M. This counts as COGS and is included in their gross margin. If we just multiply that figure by (52 / 10) and subtract out the $576M, we can see that it would decrease their annual Gross Margin by $2.5B. Add on an additional 30% of the combined $3B in salary expense since I doubt the temp workers get benefits, and you'd be looking at a total of ~$3.4 billion.
So in 2016, this would've changed their gross income from $47.7 (35% GM) billion to $44.3 billion (32.5% GM).
https://a16z.com/2014/09/05/why-amazon-has-no-profits-and-wh...