From Brotato Wiki
Shop
The shop will offer 4 random items. These items will depend on your character, 
Luck, number of waves progressed and if you have chosen to lock it. Each item has a cost depending on its rarity and their base value. You can purchase unique items once.
Mechanics
Rolling
Rolling is done buy refreshing your shop for a certain amount of currency. This amount gets increased as you roll. 
Dangerous Bunny will allow you to have +1 roll for free each round this effect stacks.
Locking
You can lock as many items as you can which ensures your next shop will keep those items of your choice. This is very useful when you are going for certain stats. This action does not cost money. If you lock an item and reroll it will keep the locked item.
Combining
You can combine weapons by having two of the same weapon equipped. This will combine them freeing up a weapon slot and upgrading the item to a higher rarity.
Also, if you have filled your weapon slots and buy an exact rarity of an item you wield that is not legendary the item will automatically combine making it a higher tier.
Recycling
You can recycle your weapons for a portion of its purchased price. This will remove the item and will grant you the amount. 
Recycling Machine will increase this amount by 35%.
Elite and swarm radar
On danger 2+ the shop will display which waves contain which waves have swarms or elites in.
Rarity of Shop Items and Luck
| Min Wave | Base Chance | +Chance per Wave | Max Chance | |
|---|---|---|---|---|
| Tier 1 | 1 | 100% | 0% | 100% |
| Tier 2 | 1 | 0% | 6% | 60% |
| Tier 3 | 3 | 0% | 2% | 25% |
| Tier 4 | 7 | 0% | 0.23% | 8% |
Calculation for chances are done by:
= ((Chance per Wave * (Current Wave - Min Wave)) + Base Chance) * (100%+Luck)
Chance per wave starts adding chance after the Min Wave.
This value cannot exceed Max Chance.
Rarities with 0 Luck
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tier 1 | 100% | 94% | 88% | 80% | 72% | 64% | 56% | 47.77% | 39.54% | 31.31% | 23.08% | 20.85% | 18.62% | 16.39% | 14.16% | 12.93% | 12.7% | 12.47% | 12.24% |
| Tier 2 | 0% | 6% | 12% | 18% | 24% | 30% | 36% | 42% | 48% | 52% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% |
| Tier 3 | 0% | 0% | 0% | 2% | 4% | 6% | 8% | 10% | 12% | 14% | 16% | 18% | 20% | 22% | 24% | 25% | 25% | 25% | 25% |
| Tier 4 | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0.23% | 0.46% | 0.69% | 0.92% | 1.15% | 1.38% | 1.61% | 1.84% | 2.07% | 2.3% | 2.53% | 2.76% |
Luck increases these chances by x%, but that still cannot exceed the Max Chance.
For example:
- In wave 10 with 0 Luck, the chance for Tier 3 is 2*7 = 14% -- extrapolated: ( 2% * ( 10 - 3 ) + 0% ) * ( 100% + 0% )
- In wave 10 with 50 Luck, it increases by 50%, 2*10.5 = 21% -- extrapolated: ( 2% * ( 10 - 3 ) + 0% ) * ( 100% + 50% )
- In wave 10 with 100 Luck, it increases by 100%, 2*14 = 28% -- extrapolated: ( 2% * ( 10 - 3 ) + 0% ) * ( 100% + 100% )
- However, Max Chance for Tier 3 is 25%, so the chance for Tier 3 ends up as 25%.
Rarities with 50 Luck
Increases chances by 50% (*1.5), up to the Max Chance.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tier 1 | 100% | 91% | 82% | 70% | 58% | 46% | 34% | 24.66% | 21.31% | 17.96% | 14.62% | 13.28% | 12.93% | 12.59% | 12.24% | 11.89% | 11.55% | 11.21% | 10.86% |
| Tier 2 | 0% | 9% | 18% | 27% | 36% | 45% | 54% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% |
| Tier 3 | 0% | 0% | 0% | 3% | 6% | 9% | 12% | 15% | 18% | 21% | 24% | 25% | 25% | 25% | 25% | 25% | 25% | 25% | 25% |
| Tier 4 | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0.35% | 0.69% | 1.04% | 1.38% | 1.73% | 2.07% | 2.42% | 2.76% | 3.11% | 3.45% | 3.8% | 4.14% |
Rarities with 100 Luck
Increases chances by 100% (*2), up to the Max Chance.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tier 1 | 100% | 88% | 76% | 60% | 44% | 28% | 24% | 19.54% | 15.08% | 13.62% | 13.16% | 12.7% | 12.24% | 11.78% | 11.32% | 10.86% | 10.4% | 9.94% | 9.48% |
| Tier 2 | 0% | 12% | 24% | 36% | 48% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% | 60% |
| Tier 3 | 0% | 0% | 0% | 4% | 8% | 12% | 16% | 20% | 24% | 25% | 25% | 25% | 25% | 25% | 25% | 25% | 25% | 25% | 25% |
| Tier 4 | 0% | 0% | 0% | 0% | 0% | 0% | 0% | 0.46% | 0.92% | 1.38% | 1.84% | 2.3% | 2.76% | 3.22% | 3.68% | 4.14% | 4.6% | 5.06% | 5.52% |
Modifiers
Item Cost
Coupon reduces shop prices by -5%, and is the only item that can do this.
Waves
Each new wave increases the chance for rarer items, as shown in Rarity of Shop Items and Luck above (see "current wave" in the calculations).
This means that harder waves give better shops.
Character Specific
Bull is the only character that will not get offered a weapon since it cannot use weapons. It will only get offered items.
Demon is a unique character that buys items with 
Max HP.
Entrepreneur has -30% Items Price buff.
Saver has +50% Items Price debuff.
Weapon Classes
Holding a weapon that belongs to a class will make other weapons in that class more likely to appear.
See Weapon Classes for more info.
Item Tags
Most items are have item tags, which group them by related properties. Certain characters have a higher chance of finding items with specific tags.
- Eg. "Less Enemies" groups items that reduce enemy spawn counts, and
Old has a higher chance of finding items with this tag.
See Item Tags for more info.