If you want it to be formal, your options are quite limited. For general use, 'regards' is by far and away the most commonly used phrase. 'yours sincerely' is not so common in email as it is in snail mail. Other options could be customised to the content of your email. Things like 'I look forward to your reply', (or ...to your order, if you want to be more pushy!) 'Please do not hesitate to contact me again' 'I hope this clarifies the matter' 'Thank you for your feedback' General statements like that can be used without a subsequent 'regards'