The recommended SOA approach is to choose coarse-grained services, which would seem to argue the minimum amount of API calls.
However, from a business point of view, item selection and purchase and item delivery are two very different concerns that should be separate components in your architecture. For item selection, you need to consider things like inventory and pricing. For delivery address, you need to consider user address lists, address validation, shipping, and taxation.
Those two components are not likely to interact much except maybe some external association between an item id and address id. For this reason, I'd recommend two API calls. Functionally, this would also allow your API users do things like update the delivery address without re-purchasing an item, send the bill to one address and the item to another address, etc.