Akasashasha:
how much it cost to them, is it cheaper if I send them plain text...do anyone have any suggestion on this point ?
I think you're missing the point.. It's not about COST, it's about CHOICE, and you have usurped your customer's choice because you have assumed that 3000 people made the wrong choice.
If you are using Zen Cart's mailing list function, it supports both plain text and HTML newsletters. You should be creating two versions when you send out newsletters anyway. If you are using another newsletter service/program it too should support both plain text and HTML. By the way there is no reason you cannot include links/URLs in both formats..
What it sounds like you are wanting to create is e-mails with CLICK-ABLE links. If that is indeed what you are after, you should know that many e-mail clients turn off URLS/links and other web content in e-mails by default anyway. This allows the recipient to determine which URLs/links and content is safe. There is NOTHING you can do to control that.
My suggestion is to follow best practices, and create text and HTML formats for all your newsletters and let your customers decide how they wish to deal with your e-mails.