Imagine two invoices where each row contains an item value and its weight. Write a function merge_items that receives two lists of pairs, where each pair contains a value and a weight, and values do not repeat within the same list. Merge the lists by adding weights for matching values and keeping unique values as they are. Return the pairs sorted by value.

def merge_items(items1: list[list[int]], items2: list[list[int]]) -> list[list[int]]: