In UK, supermarkets deliver grocery in as many bags they deem fit. The cost per bag is fixed (10p), but we won't know the total amount we'll be charged on bags until charged, because we won't know how many bags they'll use to pack our grocery in. Does this ambiguity make it haram to order from them?

Ustadh AbdulAziz al-Haqqan

But they will tell u before you pay and if that is the case it is no longer ambiguous.