I suggest adding a feature on the mogstation to explain that you cannot add/purchase additional time/days for your account if it already has time remaining from a subscription on it.

It does tell you this once you try to actually apply the days/payment directly with a debit card/crysta which is good, but there is a problem in the process...

To use crysta to purchase the additional time you have to buy the crysta using a separate system first. The separate crysta purchasing system does not warn that you cannot apply it if there is time remaining on a sub (even if the total keeps you under 180 days) and lets you go ahead and buy the crysta. So you end up purchasing time, thinking you can apply it and then are told no afterwards without having been warned beforehand. A simple explanation ahead of time in the crysta purchasing service would help fix this.