The answer will be found in the aircraft TO. There is a limit but it depends on the conditions. For instance, on the MD-11 the door limit is 40 kts (I'm pretty sure but don't have the TO handy) but if the door is on the downwind side it can be opened normally and even if the door is on the upwind side it can be partially opened. You should be able to save your payload by positioning the aircraft to put the door on the downwind side if the 737 TO allows that.