What is the best/smart solution for uploading the customer invoice on the customer portal wherein we have various customer portals and manually update the details. The objective is to make the process efficient - can you suggest any cloud based solution that can help here? Open for other suggestions as well - one thing we are exploring is to create APIs.

Are you talking about uploading your invoices into a Third Party Portal like Coupa or Taulia?

If this is a tool to help doing a two-way match to pay vendors,  I have seen a customized software solution that the vendors go to a vendor portal and enter their PO number (mainly) to generate an electronic invoice that they acknowledge. the invoice generated gets its costs from the company's PO, gets quantities from the company's receivers (whether materials or services). Training materials to help the vendors enter all the correct data in the portal helps reduce frustration when the portal rejects invoices (e.g., tax data is wrong).  This solution doesn't work for every invoice, but it goes a long way to managing the routine purchases.  The solution I mention was not cloud based when it started production.