We will be flying LIS-FRA-SFO with the FRA-SFO leg on UA (LIS-FRA is on Tap Portugal). This is an award ticket in J paid with UA miles, in case it matters. We have a 5+ hour layover in FRA and are planning to go into central Frankfurt to meet my cousin. We are contemplating getting a box from him to check onto our FRA-SFO leg. Is this possible, or will it cause some sort of difficulty? Our other bags will have been checked through to SFO in LIS. It seems like this shouldn't be an issue, but I thought I'd check with the UA experts to be sure.