From a dealer, you’ll have better warranty coverage and a physical place to bring the bike if you should need work. That’s why you pay more.
If you like to do your own work and love to learn, you can save money and have fun learning. Only, do your homework before you do any work. For some, it’s worth it to pay more and have someone else maintain your bike, because that’s easy. People like easy.
Should you choose to buy your kit from Amazon, aliexpress, or eBay. It’s all the same product. Just be sure to check the sellers/stores reviews before committing. Ask questions, if you have to.