There are not a lot of rules for writing sales letters for your online business.  But there is one supreme rule that I don't want you to forget or ignore.  That rule is...
Don't be boring.

That sounds pretty simple, doesn't it?

Don't be boring.

It's a lot tougher than it sounds. Because different audiences have different definitions of "boring."

If you are writing for a technical audience, then you need to discuss the technical details of your product. Mind you, talking about R-values of insulation will bore the hell out of most people. But if your audience is insulation installers, then you'd better be talking about that stuff. That's the reason they came to your site. So don't be afraid of details...if you audience cares about them.

And "Don't be boring" applies to your website design as well. If you are selling a product for business executives, your site better look slick and businesslike. If you are selling baby products, use pastel colors and cute fonts.

Same for the tone of your website copy. You need to use the right words and jargon. You will lose all credibility if you don't get the basic vocabulary down perfectly. Yes, that means a lot of time researching your niche, but it will be worth it.

Don't be boring.  A simple rule that is very hard to follow.