I have a number of transactions that are small FX fees for US online services, because BOI records them as a separate transaction. Is it usual practice to record these as part of the same bill as the main purchase, or use a journal entry – or something else? Thanks!

Hi Dave,
Fees are just charges for services and are classed as a standard Bill. Sometimes accountants will class them in different categories and that can get confusing. 
So you can just create a normal bill for them 
Pete
PS
If you use Stripe or Paypal we create these bills for you automatically (enormous bills!). We also update your tax if you make an FX gain as that’s classed as income. 
This is part of our eagerness for digital banking. FX can get complex (exchange, fx gain or loss, tax, fees) the end user doesn’t know nor care. Lots of numbers and % and nothing really adding up. 
Add banks hiding fees in transactions and you end up with a world of pain. 
As you well know Dave, when there is pain in banking it’s usually the bank hiding some fee!