Yes, be sure to always include an "Unsubscribe" link in any commercial email message.
Just because customers did business with you in the past does not give you the right to send them spam. Here is a quote from the US FTC:
"Despite its name, the CAN-SPAM Act doesn’t apply just to bulk email. It covers all commercial messages, which the law defines as “any electronic mail message the primary purpose of which is the commercial advertisement or promotion of a commercial product or service,” including email that promotes content on commercial websites. The law makes no exception for business-to-business email. That means all email – for example, a message to former customers announcing a new product line – must comply with the law."
Any unsolicited email is "spam". If someone reports you for spam the only good defense is a double-opt-in from them, proving that the recipient did request your message(s).
Many people report messages as spam accidentally, thinking they are just unsubscribing. Others forget that they requested your messages and report you, in spite of having opted-in previously.
Since spam reports can get your email address and/or web site blacklisted and blocked (or even get you a fine), you must be ready to protect yourself.
So, the best method is to have your customers opt-in to your mail list using a standard web form (or perhaps a paper form at your store), such that they get an autoresponder message to which they must respond to prove they want your messages ("double opt-in").
All the online mail list services provide these online forms and autoresponders. For example www.MailChimp.com or www.AWeber.com.
Hope this helps...
_jim coe