I use a micrometer!

Just a plastic bunnings one but it's good enough for measuring postage.

(and I never go near a counter with letters for fear of a slanging match with counter staff).

As for "Free" postage, I will never do it for 2 main reasons:

1. I often combine postage so it's much easier with postage as a separate item.

2. With the frequency of pricing changes I have to come up with new shipping rates 2 or 3 times a year.

    Often only one or two weight classes or types will change so only certain things need re-pricing.

    If I have the postage separate I can reprice in bulk and can sort my items by postage rate.