You can easily find your Onport invoices either through your Billing area in Onport, or your Shopify account (if you're billed through Shopify).

If you are being invoiced through Shopify, you'll be able to find any charges related to your subscription within your Shopify account. If instead you are being invoiced through a credit card, you can find these invoices through your Onport Setup area. Setup > Billing > Billing details:

From the billing area, you'll then be able to download any invoices, change your payment details or make adjustments to your plan.