Thanks for posting! This is a great feature suggestion; we're absolutely interested in making settings in SendNow (and Acrobat.com and Adobe CreatePDF) more customizable, so we'll certainly be discussing your idea as we plan for future releases.
Also, since it sounds like you've been able to use the service a bit, I'd like to invite you to participate in Adobe's Customer Reference Program; it would just consist of a 30-minute phone call with someone from the team here, so it wouldn't be a huge commitment. We'd very much appreciate to hear your feedback on the product. Please let me know if you'd be interested in speaking with us; if you are, I can have our Customer Reference Program Manager contact you shortly.
Thank you again for posting your suggestion; and keep 'em coming as you continue to use SendNow!
Acrobat.com Community Manager
I see this is a rather old post, however as a new user to the service, I would like to second this request. PLEASE make the file availability parameter a customized value.