Use attr_reader instead of shared instance variables

app/services/btcpay_manager/fetch_lightning_wallet_balance.rb

@@ -1,7 +1,7 @@
 module BtcpayManager
   class FetchLightningWalletBalance < BtcpayManagerService
     def call
-      res = get "stores/#{@store_id}/lightning/BTC/balance"
+      res = get "stores/#{store_id}/lightning/BTC/balance"
 
       {
         balance: res["offchain"]["local"].to_i / 1000 # msats to sats

app/services/btcpay_manager/fetch_onchain_wallet_balance.rb

@@ -1,7 +1,7 @@
 module BtcpayManager
   class FetchOnchainWalletBalance < BtcpayManagerService
     def call
-      res = get "stores/#{@store_id}/payment-methods/onchain/BTC/wallet"
+      res = get "stores/#{store_id}/payment-methods/onchain/BTC/wallet"
 
       {
         balance: (res["balance"].to_f * 100000000).to_i, # BTC to sats

app/services/btcpay_manager_service.rb

@@ -2,6 +2,8 @@
 # API Docs: https://docs.btcpayserver.org/API/Greenfield/v1/
 #
 class BtcpayManagerService < ApplicationService
+  attr_reader :base_url, :store_id, :auth_token
+
   def initialize
     @base_url   = Setting.btcpay_api_url
     @store_id   = Setting.btcpay_store_id
@@ -11,10 +13,10 @@ class BtcpayManagerService < ApplicationService
   private
 
     def get(endpoint)
-      res = Faraday.get("#{@base_url}/#{endpoint}", {}, {
+      res = Faraday.get("#{base_url}/#{endpoint}", {}, {
         "Content-Type" => "application/json",
         "Accept" => "application/json",
-        "Authorization" => "token #{@auth_token}"
+        "Authorization" => "token #{auth_token}"
       })
 
       JSON.parse(res.body)