After you upload your first bill receipt, Pebble processes the payment exactly two weeks later. Once processed, it may take a few additional weeks for the tradeline to appear on your credit report, as credit bureaus have different update cycles.
While a new tradeline might initially cause a temporary decrease in your score, Pebble will continue to report your receipts along with the reporting fee. Over time, consistent reporting of your bills and timely payments of the reporting fee should help build tangible credit data for you.