Could you please add the amount for this month to the invoice for next month because it's not much this month?
Could I ask you to include this month's invoice with next month's as it is not very much this month?
1) Could you please add the amount for this month to the invoice for next month because it's not much this month?
「今月の請求額が少ないので、今月の請求額を来月分と合わせて払ってもらえますか？」
「請求額が少額のため」は because it's not much this month と言えます。ここの it は the amount for this month を指しています。
2) Could I ask you to include this month's invoice with next month's as it is not very much this month?
「今月分は少額なので、今月の請求書を来月分に含めてもらえますか？」
「請求額が少額なので」は as it is not very much this month とも言えます。ここの it は (this month's) invoice を指しています。
ご参考になれば幸いです！
日本 
