The following seems wrong and confusing to me:
Gifting is not available for split orders. Because you already have items in your cart from Zanybobs shop, adding items from different stores will not make gifting possible
Is there a reason why "adding items from different stores will not make gifting possible" seems confusing, or is it just confusing to me.
Is there a better way to say this?
You're right that it doesn't sound correct. The 'not' modifier is in an awkward place and should be attached to the word it is changing (possible).
...adding items from different stores will not make gifting possible
should be
...adding items from different stores will make gifting not possible
This sentence not is correct, because it means that gifting is not possible and will still not be possible after adding items. You will want to move "not" (with "not possible" becoming "impossible") :
Because [...] shop, adding items from different stores will make gifting impossible.
Or you could reformulate :
Because [...] shop, adding items from different stores will disable gifting.
You are correct. That is quite an awkward sentence, as it could be interpreted as adding items does not necessarily make gifting possible. It could also be interpreted as adding items makes gifting unavailable.
