I would suggest a responsive solution, by using a media query in css.
Example of a media query in the css file, using a class which will only show on screens smaller than 500px:
@media screen and (min-width: 500px) {
.mobile {display: none;}}
Then add the class "mobile" to the website section or div which is intended for mobile visitors and you're done. The offer will only be visible for visitors on small screens. Of course you can adjust the width and also add more of these criteria for eg small phones, regular phones, tablets etc.
"Depend on the rabbit's foot if you like, but remember it didn't work for the rabbit." R.E. Shay