When you using the app, it's best if you claim the offer before you make your purchase. There are a few reasons why we ask you to claim the offer before you make your purchase.
Gas stations in Upside are changing their offers all the time based on their business needs at the moment you open the app. Gas prices and margins are changing constantly, so the offers in Upside change along with it. We ask you to claim before purchase because if you don't claim it, the offer may change by the time you make your purchase. We don't want you getting a worse offer later on because it wasn't claimed before you went to shop.
Gas stations provide offers to get you to go to their station and the offer amounts are based on their real-time needs. They want to see that the offer is what drove your business to them. Submitting a receipt for a past receipt doesn't work because your decision of where to buy gas was already made. Additionally, the gas station's business needs at that past time may have been different so the offer you claimed now would be money losing for them.
A similar example would be if you went to a restaurant for dinner after happy hour ended, but asked to pay happy hour prices. The restaurant offered happy hour specials, because they were trying to drive more business during those hours. After the happy hour ends, they don't honor happy hour specials anymore because they are busy and don't need to discount to earn your business.