To download the invoices for your Postcron subscription click on Your Account, at the bottom of the side menu where your profile picture is, and then click on the Billing option.
In the last section, Invoices, you will find all the invoices listed and ordered by date: you can download each one as a .pdf file simply by clicking the Download button.
You can enter or edit your billing information in the Billing Info section. Remember to save the changes!