I am considering redoing my WordPress website by rewriting myself to get something a little lighter weight. The only problem I see myself getting into is rewriting my WooCommerce section. I certainly don't want anyone getting in danger. I will only be using Paypal but we still have to collect shipping information.
Is it too much of a security risk to write my own online store?
Simple answer: Unless you are an expert, it is generally too risky to write your own anything. Well, you can write stuff, just don't use it or publish it for others to use unless you're damn sure it's secure. Since you're asking, I'm going to assume you're not an expert.
The only problem I see myself getting into is rewriting my WooCommerce section. I certainly don't want anyone getting in danger. I will only be using Paypal but we still have to collect shipping information. Any thoughts?
I can't speak to the security features of WooCommerce, as I've never audited a WooCommerce project as of this writing.
Unless you can do it better than what's available, use what's available. They might have prevented vulnerabilities that you would never even have thought could exist.
However, if you're doing it for your own educational purposes, please do experiment with your own solutions. You might find a better way to implement a feature than what already exists and possibly not realize it.
As long as you make it plain and clear that the code is experimental, and you don't actually deploy it to a production system without getting examined by a security expert, the "don't write your own" advice can be ignored.