Yes. Virgin Mobile answered about your Auto Top-Up question in the FAQs:
Auto Top-Up for Pay by the Minute plans
Auto Top-Up is sort of like Virgin Mobile's valet service.
Register a credit or debit card or PayPal account, choose which way you'd like to Auto Top-Up, and we'll automatically add airtime to your account based on the settings you've chosen.
Just pick an amount - $10, $15 ,$20, $25, $30, $35, $40, $50, or $60 - and we'll add it to your account
(1) once a month
(2) once every 90 days
(3) whenever your balance drops below $5 or 90 days since your last Top-Up, whichever comes first.
It's your choice. And you can change your Auto Top-Up settings anytime through our website www.virginmobileusa.com.
Of course, you can always register a credit/debit card or PayPal account and do Top-Ups on your own - from our Web site or right from your phone.
You'll need to save your credit or debit card info inside your payLo account. To edit your CREDIT CARD SETTINGS, Sign In to your account on the Virgin Mobile site. Click on the Setting & Preference tab. Click EDIT CREDIT CARD SETTINGS.
To edit your Auto-Topup Settings, Sign In to your account on the Virgin Mobile site, click on the Setting & Preference tab. Under CHANGE AUTO TOP-UP SETTINGS click // EDIT NOW.