Invitation letter for in-laws for B2 visa

[sunitadha]

Hello,
Can somebody please tell me...do I have to send Invitation letter by courier to India to invite my in-laws or is it ok if I send email and my in-laws take print out of the same?

Suggest me the way to send invitation letter...by courier, by email...or scan the invitation letter which has my signature and send by email.

Thanks.

[immihelp]

Original letter by good old regular mail.