Support varies from dev to dev. Some will offer customization at an additional cost; some will direct you to do it yourself, or hire a programmer. Some you will never get a response from. Once you download the plugin you have access to the code for customization, but if you don't have the skills needed to do the customization, you may be stuck.
1. Look at version compatibility before purchasing. Purchasing plugins that do not support the current Oxwall version is not advisable. This is a good sign that the dev is not, or no longer updating their plugin, and it may not function properly.
2. Try to contact the dev either through the plugin forum, or PM. If there is no response in a reasonable time frame, the dev may have abandoned the plugin. If the version is up to date, and you get no response, that will give a strong indication of what seeking support will be like.
3. Read the plugin forums to see how well the dev responds, and see others reviews of the plugin.