Letter credits are available at the following rates:
10 Credits: £15 + VAT (£1.5 per credit)
30 Credits: £33 + VAT (£1.1 per credit)
50 Credits: £50 + VAT (£1 per credit)
100 Credits: £90 + VAT (£0.9 per credit)
500 Credits: £425 + VAT (£0.85 per credit)
1000 Credits: £800 + VAT (£0.8 per credit)
2500 Credits: £1875 + VAT (£0.75 per credit)
5000 Credits: £3500 + VAT (£0.7 per credit)
Sending a standard letter costs 1 credit.
If you need to add an extra page to your letter, it's only 0.25 credits.
Sending a letter + Google outline (Legacy) feature will cost you 1.25 credits.