"15 minutes could save you 15% of more on car insurance" is one of the most powerful closes ever, used by Geico insurance. they tell the customer:
a) what they have to do
b) that it won't take very long or be a hassle
c) what they can expect as a potential outcome
The end of your sales letter should do the same. After you build value you have to give the customer a hassle-free avenue to pursue what you have to offer. You can even offer your customer something they don't have to pay for as a way to get them to call.
Example: "calll ***-***-**** and get a free 10-minute audit of your site's SEO. find out how we can do for you what we've already done for hundreds of businesses - boost revenues and crush sales targets! hope to hear from you!"
this is obviously a bit longer and less catchy, but it fills the 3 categories above and making the customer feel comfortable about taking the next step.
I don't personally use phrases like "act now" because i think a more specific call to action gets better results, like "CALL *phone number* to *get THIS*. here you aren't just telling the customer to act, you're telling them how to act and what to expect from their action which is very important.