Final answer:
The unexpected surprise in the mail is not explicitly stated, but it implies various possibilities including a special package, an emotional revelation, or significant personal items.
Step-by-step explanation:
The unexpected surprise that arrived in the mail for mother in the provided texts is varied and not explicitly stated. Based on the excerpts, it is suggested that the surprising items ranged from a package that might be related to Mrs. Boldin and a lace, a bundle of emotion and revelation of love, a referenced gift in a letter, to a significant personal or emotional value item. In one of the extracts, we see McTeague receiving a packing-case with something he wasn't expecting, which could imply that similar unexpected parcels or items might have arrived for the mother in the story.
Given the various texts, the prize or surprise could be a physical gift, such as the one Trina gave to McTeague, or an emotional surprise such as a letter of love and appreciation. The actual surprise varies and depends on the context of each individual's story or letter within the provided excerpts.