A range of internal and external sites are available for a variety of different promotions including car displays as well as home-improvement products and services.
Space & People chief executive officer Matthew Bending said: "This contract will provide an ideal platform for brands and their key consumers to meet."
Meanwhile, Dobbies, which also works with Glasgow-based Space & People, has changed its mind about charging charities for promotional space, a practice that is "common" among many retailers.
A Dobbies representative confirmed: "Dobbies will not be charging charities for renting promotional space in our garden centres."
Marketing and insight manager Dawn Clarke added: "It is common practice for national charities to pay for space. A lot of venues have their own policy on local charities.
"The charity industry has become more professionalised. They know that if they book it they have access to the higher footfall weeks that they want rather than relying on ad hoc."
Clarke pointed out that garden centres are "high-footfall venues".
Garden centres can charge charities up to £400 a week, or £200 for weekends, to collect donations in its stores.