You should definitely charge more money for your app. Like double and it would still be worth it if for nothing else but your excellent customer service and your dedication. Or you can include tip button via in-app purchase in the settings page so if someone wants to give you some more money they can.
I think it is dangerous to charge more up front, for new user uptake, even though it is worth it. But I have also wondered how to pay more, to support development, without turning into subscription which I loath. I would definitely tip, but other options would in app purchase. Maybe unlock specialized/advanced variants, because I fear it will get feature creep, to satisfy everyone, making the user interface cumbersome or confusing for simple users.
Thanks to you both for your suggestions. I considered a tip jar but it means an in-app-purchase, which would be mentioned on the App Store and can put people off. When users ask if they can tip (which is always great to hear!) then I usually ask if they can show their appreciation by spreading the word about the app.
At the moment the app pays the bills (which was a great milestone to reach) but not much more than that. However sales are slowly increasing so I am optimistic.
I must admit that a subscription is a possibility in the future, although I know that it would put a lot of people off. The problem is that I want to add extra features that would cost me money every time they are used and I cannot do that without having a subscription.
At the moment every time a user downloads a map then it costs me money. I can cover this with the current one-off payment model (although it doesn't help that the company supplying the data that I use to draw the maps is about to raise its prices by a factor of x2.5!). However I can't afford to add other features that would also cost money, such as topo maps, satellite maps, turn-by-turn navigation, weather reports etc.
So I am considering a subscription just for features like that. All the existing features would still come in the one-off price but features that cost money as they are used would require a subscription. It would probably be as cheap as possible though, because I wouldn't want to put people off too much.
However this is all a long way off because I am trying to increase the user base at the moment and subscriptions are very off-putting. It just means that some of the features that people want to see are also a long way off.