You are talking about trying to buy an increase in website rank, right? I believe most of those providers are scammers or they use techniques that search engines actively detect and downgrade.
Google says:"Sites' positions in our search results are determined based on hundreds of factors designed to provide end-users with helpful, accurate search results. In general, webmasters can improve the rank of their sites by creating high-quality sites that users will want to use and share." They provide some tools and tutorials on how best to get noticed by their crawler.
You can also purchase advertising with Google and other entities to display when certain search words are use. That may be a more effective use of your budget than some fly-by-night SEO optimizer trying to game the system.