This stomps on `defaultInvoiceItemParams` which is undesirable.

I would recommend creating a new map and using putAll, or possibly just making a helper method `getInvoiceItemParams(String customer)` in the style of `getUniqueCouponParams`:
```java
	static Map<String, Object> getUniqueCouponParams() {
		Map<String, Object> uniqueParams = new HashMap<String, Object>();
		uniqueParams.putAll(defaultCouponParams);
		uniqueParams.put("id", getUniqueCouponId());
		return uniqueParams;
	}
```