Which service does not offer a guaranteed delivery timeframe?

The reason Priority Mail is the correct answer in this context is that it does not provide a guaranteed delivery timeframe like the other services mentioned. Priority Mail aims to deliver packages within a certain time frame—commonly 1 to 3 business days—but it does not come with a guarantee. This means that while your package is expected to arrive within that window, there can still be unforeseen delays that might extend the delivery time.

In contrast, both Priority Mail Express and Global Express Guarantee do offer guaranteed delivery. Priority Mail Express promises overnight delivery to most locations, making it a reliable option for urgent shipments. Similarly, Global Express Guarantee provides guaranteed international delivery in specific time frames. First Class Mail typically has expected delivery times as well but is primarily intended for letters and small parcels and does not guarantee delivery either, which is why it’s important to note that the emphasis on guaranteed service set Priority Mail apart from the other two options in the context of this question.