When you resuse envelopes how do you hide the postage paid part?  Like the part the post office puts in the upper corner?  Do you marker over it?  My label is never big enough to cover the whole thing. 

I just try to peel it off if I can. If not, then I just mark it out with a marker. Some postal employees get irritated if it's not covered.

I peel off the old labels and old postage. If there is a tricky one, white stickers work. 

My postal worker usually just applies the new one over the existing one. As long as it’s covered one way or another, All is good.

I try to peel it off. If it won't budge, I use a black Sharpie & try to scratch it out the best I can.

I tape blank white paper over it.

I use labels if i have them. Or brown clear packing tape I get at Walmart 

I use brown packing tape to cover the old address and postage markings.  It can't be pealed off with damaging what is underneath.