@willisbob Once you decide what roaming add ons you need you can top up your balance in the account with the approximate funds you will need. The roaming add ons start immediately and run on a 24 hour clock so purchasing them right before you need them is best. At the airport on WiFi once you land or on mobile or WiFi data just before you cross the border (while you're waiting in the line up.)
Consider what has already been mentioned....you will continue to receive text messages. You may also receive phone calls. You will not be able to send texts or make calls unless you have an app like text now that you can use on WiFi or a roaming add on. How much data do you regularly use in a month? Decide if you can get by on the minimum offered (250mb) and an app for calling and texting. You may not need a texting or calling roaming add on at all.
You could take the $20/1gb data add on for the first 10 days if you think you need it...and see how much free WiFi you use compared to mobile data and adjust your second 10 day add on to suit your needs. You may find you can get away with 2x$10/250mb 10 day add ons. The later in the day your add on expires the better compared to when you will be on free hotel (your accommodations) Wi-Fi. So when your first add on expires you're covered on WiFi til the ability to repurchase happens sometime after midnight et. You will want to purchase the second add on just before you leave free WiFi the next day...9-10am? This will stretch 2×10 day add ons to cover 21 days of your travel.
