South Carolina sales tax cut for people age 85 and older

Fixes PolicyEngine#5387

policyengine_us/tests/policy/baseline/gov/states/sc/tax/sales_and_use/sc_sales_and_use_exclusion_eligible.yaml

@@ -0,0 +1,15 @@
+- name: Person age 85 or above eligible for tax exclusion
+  period: 2022
+  input:
+    age: 90
+    state_code: SC
+  output:
+    sc_sales_and_use_exclusion_eligible: true
+
+- name: Person age under 85 not eligible for tax exclusion
+  period: 2022
+  input:
+    age: 70
+    state_code: SC
+  output:
+    sc_sales_and_use_exclusion_eligible: false

policyengine_us/tests/policy/baseline/gov/states/sc/tax/sales_and_use/sc_sales_and_use_tax.yaml

@@ -0,0 +1,17 @@
+- name: Person age 85 or above eligible for tax exclusion
+  period: 2022
+  input:
+    sc_sales_and_use_exclusion_eligible: true
+    sc_sales_and_purchases_proceeds: 10_000
+    state_code: SC
+  output:
+    sc_sales_and_use_tax: 500
+
+- name: Person age under 85 not eligible for tax exclusion
+  period: 2022
+  input:
+    sc_sales_and_use_exclusion_eligible: false
+    sc_sales_and_purchases_proceeds: 10_000
+    state_code: SC
+  output:
+    sc_sales_and_use_tax: 600

policyengine_us/variables/gov/states/sc/tax/sales_and_use/sc_sales_and_purchases_proceeds.py

@@ -0,0 +1,9 @@
+from policyengine_us.model_api import *
+
+
+class sc_sales_and_purchases_proceeds(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "South Carolina sales and purchases proceeds"
+    unit = USD
+    definition_period = YEAR

policyengine_us/variables/gov/states/sc/tax/sales_and_use/sc_sales_and_use_tax.py

@@ -6,17 +6,22 @@ class sc_sales_and_use_tax(Variable):
     entity = TaxUnit
     label = "South Carolina sales and use tax"
     unit = USD
+    reference = "https://dor.sc.gov/forms-site/Forms/ST3.pdf#page=2"
     definition_period = YEAR
     defined_for = StateCode.SC
 
     def formula(tax_unit, period, parameters):
         p = parameters(period).gov.states.sc.tax.sales_and_use
 
         # base amount
+        taxable_sales_and_purchases = tax_unit(
+            "sc_sales_and_purchases_proceeds", period
+        )
 
         # sales and use tax rate with eligible exclusion
         eligible = tax_unit("sc_sales_and_use_exclusion_eligible", period)
         exclusion = p.exclusion.percentage * eligible
         rate_applied = p.general - exclusion
 
         # return base amount * rate_applied
+        return taxable_sales_and_purchases * rate_applied