You can batch-process invoices by following the simple steps below.
Step 1: Open Simple Invoice from your Shopify store’s “Apps” section.
Step 2: Select the orders you wish to process and select one of the process options such as print, download, send, upload, and convert.
Important Note: You can process 50 invoices at once. The 50-order limit is something that Shopify controls, and apps cannot change that limit. Also, if you wish to batch-send the invoices, they are not sent simultaneously, they are added to the queue and sent sequentially.
Even processing 50 invoices at once may seem slow. If you add additional data, it may take too long to process or it may not print the information as you expected. The most common solution for that is to hide the product images. It may speed up the whole process at once. After turning off product pictures, if you are still having issues in the batch process, you can try to turn off product variants or additional product details like meta fields or properties.