All major credit and debit cards, PayPal and Apple Pay.
Invoices are available when you sign in to your account. Under Settings there is a section to show invoices.
Yes, all files needed for the courses are provided to download.
After setting up your account and signing in, you will see a community tab. If it's your first time click join. If you are part of a course that has a private forum, after you join the general community you will be added to the private forum within 24 hours. You will see it available in the side bar of the forum once accepted.