market-pricing | npm Javascript package of algorithms | Cryptocurrency library

So i'm currently working on some pricing logic where the situations are:

• 1 glove costs $2.50, 2 gloves costs $4, therefore applying a 20% discount.
• If person buys 3 gloves then it should be the $4 + original price of $2.50 making total $6.50
• If person buys 4 gloves then it should be $8.

• 1 gum costs $0.65, you can buy 6 for the price of 5, so $3.25 for 6 instead of $3.90
• If person buys 7, then it should be 6 for $3.25 + original price of $0.65 totalling $3.90
• If person buys 8, then it should be 6 for $3.25 + (original price of $0.65 * 2) totalling $4.55

So it's very important at what index/stage that the discount needs to be applied by

I took my logic inspiration from this article: http://codekata.com/kata/kata01-supermarket-pricing/

I took my code inspiration from this article: https://github.com/raddanesh/Kata01

Specifically the VolumePricingStrategy as show in this diagram:

Here is my code attempt given the logic i want to achieve: