### Available Hints:

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The card with the postman includes hints as to what you're supposed to do here.

Each of the cards with packages includes a stamp. Some of the stamps have more postage than they need, and some don't have enough.

The solution to this puzzle is what goes on the purple stamp marked ???

The four packages that have stamps with numbers give clues as to how much it costs to ship packages.

The cost to ship is related to package size. Packages will ship (indicated with a green checkmark) as long as they have at least the necessary postage, but the ones shown all have extra postage they don't need.

You can determine the package size by figuring out how many blocks the package contains. There are no hidden gaps in the packages. The smallest package is one block.

The cost to ship increases linearly with package size. A package with three blocks costs three times as much to ship as a package with one block. That's what the 'What's your uniform cost?' hint refers to. The cost per block is an integer (no decimals or fractions).

The packages that ship (with green checkmarks) have postage that's more than enough to ship. If you figure out how much postage per block has been applied, you can figure out what cost per block is higher than needed. The packages that don't ship (with red X's) have a cost per block too low to ship. Somewhere in the middle is the true cost per block

The single block package ships for 20, so the maximum possible cost per block is 20. The three-block package does not ship for 41, so the minimum cost per block is at least 14 (14*3 would be 42, but 41 doesn't work). The four-block package ships for 71, so the maximum cost per block is less than 18 (18*4 = 72). The six-block package does not ship for 99, so the minimum cost per block is more than 99/6 = 16.5. If the maximum cost per block is less than 18 and the minimum is more than 16.5, then the cost per block must be 17. The final package contains 17 blocks, so the solution is 17x17.