You mean:
1. Donate 15 minutes, which at a Amazon warehouse rate about $20/h, is about $5.
2. Donate 30 minutes, or about $10.
Worker speed affects both 1. Latency (How fast you get it), but also 2. Throughput (How much packages Amazon sends per worker/hour)
Are you seriously willing to donate that much on a regular basis? Do you think ordinary people will want to do so? If so, such a button makes sense.