Depends where you live.
Virgin Mobile answered your question in the FAQs above:
You're charged sales tax on Top-Ups, purchases of equipment and accessories, and shipping & handling charges where required by law. The amount of tax is based on the billing address of the debit/credit card or PayPal account you use.
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For the most accurate answer click 'Support' at the top of this page and ask Virgin Mobile's Customer Care via phone or email. You can dial *86 on your Virgin Mobile phone, too. You can also click LIVE CHAT on the left side of this web page, if available.
**The $20/month plan provides 400 minutes of domestic calling every month. Text messages cost 15¢ each. Charges for text messages come out of your Virgin Mobile account. To see or add to the balance in your Virgin Mobile account on your phone select MY ACCOUNT on the menu. You can also click TOP-UP at the top of this web page. Use a Virgin Mobile Top-Up card(s), credit/debit card, or PayPal account to add money to your Virgin Mobile account before you send/receive text messages.