|If you have previously purchased and downloaded 2Do, tapping on the Restore button under Settings > Upgrade to Pro should automatically upgrade you to Pro. It does so by contacting the iTunes server to validate your purchase. Remember: developers have absolutely no control over transactions and payments on iOS / Mac App Stores. Everything gets handled by the iTunes store. Previous downloads made during a free promotional period or otherwise automatically get validated as the original digital receipt is kept and stored by iTunes.
In case this isn't working for you, the only three cases where this would fail: 1) You either purchased the app using a different Apple ID (or someone else used their Apple ID to download it for you - this applies to family sharing as well, more on this below); 2) You've since moved to a different country, or 3) Your device is Jail-broken (in which case, simply disable the jailbreak and try again). In the first two cases cases, sadly the only option left is to switch to the Apple ID you originally used in order to validate the purchase, or re-purchase the in-app purchase. We are unable to help in such situations as the app will only unlock its pro features if a valid license is found.
As much as this hurts, Apple sadly enforces this in case you switch to a different country even when using the same Apple ID. All previous purchases are invalidated and rendered useless (unless you don't delete the app in between or migrate to a different device) - I've personally experienced this and lost hundreds of dollars worth of purchases, so I know how it feels. Developers in such situations are unable to help, and iTunes customer support simply points you to their existing policy on this. These constraints are laid out by Apple and there's very little developers can do. Another option of course is to wait for a price-drop / promo, but of course there are no guarantees for this either.
Regarding Family sharing in particular - this really is an Apple policy that developers can't do anything about. We work around the policies laid out by Apple and as developers we have no control over this. The only workaround we could suggest is: make sure your copy of 2Do is synced / backed-up, then uninstall 2Do. Sign into your own Apple ID (or the original Apple ID that was used to purchase 2Do), re-install 2Do, and finally try and Restore again (from Settings > Upgrade to Pro). Once that works, setup sync as before and re-sync everything back into 2Do. This procedure will ensure you don't pay again. Again, I'm incredibly sorry you have to do all this, I wish there was an easier way out for family sharing. You could of course help by leaving feedback with Apple to consider adding in-app purchases to Family Sharing as well. We're certainly not trying to be greedy here.
In case you're wondering, promo codes that Apple provides developers are only a handful and reserved for press, marketing and promotional give-aways and sadly cannot be relied upon. We simply don't have enough to cater a large number of users running into such issues. Feel free to contact us to discuss further if you've got concerns, however please be aware we're mostly unable to help in such situations. Again, sincere apologies if this has affected you, but we've tried to keep the updates to 2Do free past 8 years and there are certain situations we cannot avoid.