Each item in the basket should be regarded as representing either itself or a group of very similar items. While technically an avocado is a fruit, it can scarcely be regarded as so similar to peaches or nectarines, from the average consumer's viewpoint, that it can be regarded as representing them. Thus plums are now effectively representing all stoned fruit.
There is of course plenty of precedent for having items in the basket that are regarded as unavailable at certain times of the year. Nobody would suggest removing all such items.