Yesterdays

In accordance with Amazon API documentation ASIN is not offer ID:

An offer listing ID is an alphanumeric token that uniquely identifies an item. Use the OfferListingId instead of an item's ASIN to add the item to the cart.

Type: String
Default: None
Valid Values: An Offer Listing ID
Constraint: Required if ASIN is not used.

So you need to find way to obtain offer_id. But I can't see this functionality in python-amazon-simple-product-api library.

UPD: There is pull request