Whether this was written in PHP, .NET or Ruby.
"simple checkout store" - brilliant.
From the sounds of it we are all devs here. And, as experienced devs, we all know that saying "you just have to ..." will be your famous last words.
Adding more servers, as many of you will know, only works up to a point. From there you have to further build out databases, redis, memcache, load balancers, etc. As much as we like to fantasize about a "go faster" button which just adds more compute, often the amount of memory or CPU you have isn't the bottleneck.
When it comes to sharding, you're not wrong. Sharding off of a primary key is something that is easy to grasp. Hell, it's not overly special (especially these days). However, that doesn't mean it's easy.
If you think building a commerce platform is easy, I'd encourage you to apply. Maybe you have the type of skills we are looking for?