The validity period of the pack purchased will be accumulated accordingly if it is of the same pack. For example, if you make a purchase of Family Pack 3 days and Family Pack 30 days, the duration will be stacked up and your validity period will be 33 days.
However, the extension does not apply to two different packs. For example, if you make a purchase of Family Pack 3 days and Kids Explorer Pack 30 days, the duration will not be stacked up.