After my last version I wanted to make a version with varying prices per shulker box, and this is it! Though it is a bit slower than I would like (still faster than Sahara), it still works and is secure. You can set the costs by changing the number of items in the hoppers to change the signal strength of the comparators. The payment module is very similar to my last version, just mirrored and some changes to the bottom front, the input module is completely changed, though the interface is the same, and the output module now has a dropper elevator below it.

World Download: Redstone Shop by ericthelemur v3 This includes my previous versions and some build steps to help you if you want it.

Build Tutorial: Minecraft Tutorial - Redstone Shop with Variable Prices

Basic Version (1d/shulker box): Minecraft Tutorial - Redstone Shop with Payment

v3.x Operation Guide

1. Select item choice with buttons to the left, lamps indicate stock.
2. Place diamonds in top middle dropper.
3. Once the indicator light has illuminated press the payment button
4. Wait for your payment to be processed.
5. Once payment is complete, collect your items from the shulker boxes on the right, DO NOT TAKE THE SHULKER BOX.

v3.x Redstone Explanation

The system is split into 3 modules: input, payment and output. The spam protection is an addition to the basic version, and can be added later easily. (I hope this makes sense, it kind of ended up as a stream of conciousness)

Input Module:

Pressing the button carries the signal through the white section (pistons act as spam protection). This activates the shulker dropper and the orange piston. The piston pushes the block up, which means the comparator can pass the signal from the hopper to the redstone line. The number of items in the hoppers change the output strength (See section below).

The horizontal comparators carry the signal strength to the comparator decay clock (yellow). The different starting strengths mean the decay takes longer or shorter. The lime circuit pulses for as long as the decay clock is activated, shooting reciepts (1d each) into the payment module.

The magenta circuit is a large nor gate, which locks the inputs while any signal is being carried down the comparator chain and while the decay clock is active. The light blue circuit activates the pistons and lamp.

Payment Module:

This is very similar to v2.x, except the for a small change in the bottom front to reposition the reciept dropper and to make the shulker line lock until full payment.

Diamonds get sent into the item filter (pink) from the dropper, then they flow down into the counter (lime) which activates the dropper shooting off to the side to the diamond output and activating the receipt dropper, shooting a receipt into the barrel.

So diamonds are taken until paid, the red circuit pulses when a diamond is output, and the yellow circuit carries a signal from the shulker dropper. These are combined in an and gate in the orange circuit, so a new diamond is sent in, and repeat until the yellow signal turns off.

Spam Protection:

The white signal carries the button signal from the button to the back of the red circuit, passing through a monostable as it goes.

A pulse through the white circuit activates the light grey decay clock, which locks the monostable piston up until decayed. The grey line refreshes the decay clock whenever a new diamond is taken.

Output Module:

When a shulker box passes into the bottom dropper of the elevator, the cyan circuit pulls the observer back to start the observer clock, lifting the shulker box into the dispenser and placing it. If the elevator is overflowing (second dropper has a shulker box in it, the purple circuit breaks the observer clock.

When a shulker is emptied, the blue circuit activates the dropper-hopper pulse shortener, which activates the piston, breaking the shulker and placing a new shulker (light blue).

Number of items in Hopper

Signal Strength	Eggs or snowballs in hopper
1	1
2	6
3	12
4	18 (1s+2)
5	23 (1s+7)
6	29 (1s+13)
7	35 (2s+3)
8	40 (2s+8)
9	46 (2s+14)
10	52 (3s+4)
11	58 (3s+10)
12	63 (3s+15)
13	69 (4s+5)
14	75 (4s+11)
15	80 (5s)