You'll get rejected. Even Stephen King gets rejected. The guidelines will tell you what the editor will and will not buy, what length she wants, and how much she pays. This will save you time and postage. Although the magazine sadly ceased publication soon after Marion's death in 1999, the site still gives excellent advice.
Cover letters are another matter.
The Writers Market has this contest in every issue for the best cover letter, and the winners they publish just floor me. They're always too long and explain way more than any editor I've met wants to know. One went into great detail about the amount of research the author had done before writing the story. Long cover letters are the mark of an amateur anyway. I hope you can use it Wonderful Magazine.