How can I connect Microsoft SQL Server to MySQL
Yes, Linked server and OPENQUERY is the right approach.
However, 5k of sales per day is already a good website. If you are already on MSSQL why don't you use MSSQL for your website? Because you are using WordPress? There are solutions for that:

Project Nami: It has a few years now so it's quite stable and keeps baing developed. You can also deploy it to Azure quite simply. It might be a good alternative if you are already on MSSQL and if you want to stay there.
Ziper: I've used it in the past, try to give it a go.
Skyvia: It says it's free up to 5k per month so it really looks like your scenario. You can use it for free now and pay later if you have more than 5k of records per day.

Or if MSSQL and MySQL is what you want and you are good in usingg Microsoft technologies you can use SSIS to update data from MySQL to MSSQL and vice versa.