It is common for people to sign up to great deals advertised only to find out after doing more research that the deal offered was not as good as it seemed at first glance.
In sales there are advertised rates and actual rates. I pass a hotel every day that has a sign saying rooms from £25 when their rack rate is £125. So what are the chances of me getting a room at £25.
Many online printers offer full colour business cards for under £5 per 100. However, by the time you get to the actual check out you are over £25 with all the ad-ons.
Its the Ryan Air principle. Return flights £30. Great price. Now do you want a seat with it.