There is a limit on Shopify when the merchant passes the 50,000 variants in total. https://d.pr/i/6TC65i
The limit does not relate to the Expressfy app.
If you have more than 50,000 variants in your store, you are limited to importing 1000 variants per day.
The only way to be able to import unlimited products to Shopify is to upgrade your account to Shopify plus.