Can I use the Inserit modules in multiple sites paying just one subscription fee?
You can certainly manage as many sites as you want under the same account without having to pay the fee each time. You can either be in charge of updating the site contents of your clients yourself (and so you won’t need to create multiple accounts) or else have each one of your clients update their own contents themselves (in which case it’d be a good idea to use separate accounts for each client so that each one of them could only access their own site).
