A change since 11-May when I last topped up, it seems Fastmail no longer take lump sum prepayments against your account. Being a non-USD subscriber I had been watching exchange rates and prepaying an amount when favourable, but it seems I can do this no more.
The suggestion is to switch to a multi-year subscription but the longest I’m offered on my plan is 12 months.
This also removes one idea (discussed elsewhere on this board) for prolonging the existence of your account while others deal with your effects after your demise.