From the invoice itself, by email with the PDF attached, or by sharing a public link your client opens without signing up. The link is available on all three plans and you can switch it off whenever you want.
And if none of this was your case?
If you are already a customer, write from inside the app: it arrives with your company attached and gets solved sooner. If you are not one yet, or you want to talk about a migration, about pricing or to the press, that is what the contact form is for.