I'm only on my second tub of Stirling soap and from my limited experience it's the best soap I've ever used. Recommend it 100%.
In terms of these new instalment offerings I have used one called "LayBuy" for a non-shaving purchase (£30) - this was for six instalments. Easy/fast setup. But I found it a bit of a faff because contrary to what I expected the instalment weren't taken automatically so after the second one I just paid down the lot. I just wanted to see how it works - other companies may use different approaches.