TBH, that's what I thought too - and yet over a year later, it's been faultless <shrug> For a lot of sole-traders, most 'invoicing and accounts'-type apps are overkill, and like yourself I'm leery of subscription-based services. Trouble is, we may only produce a few dozen invoices or estimates a year and only to a few dozen clients, so we don't want to pay much - who's going to create an app that caters to that demographic, that isn"t subscription-based?
Quick Sale is the best I've found. £20, you own it, it's yours forever. They do occasional free trials - it's well worth a look, seriously.
Be interested to hear what you decide on - keep us posted.