Yes, sellers do not have to reduce postage for multiple purchases, even if they cost less to post together than if posted separately.  However, if lets say, 2 items together are just over the 500g, they cannot go together in 500g satchel.  Or couple of very light but relatively large items may be under 500g, but will not fit in the 500g satchel.  Unfortunately, it is not always possible to judge if postage is reasonable or not.  And of-course, you need to allow for cost of boxes and other packaging materials before deciding if the charge is too much.