I'm plannig to stay three days in France and then three days in Spain. If I buy $19 Prepay roaming add-on while in France will I be able continue to use it when move to Spain or would I have to buy another one?
No need. You can use your roaming add on to france going to spain since the both countries are belongs to one zone only. Just kindly make sure that you have enough entitlements for your roaming add on to use it. Cheers!