还是付了吧，要不忘了。了 is ...?
I think both denote the completion as defined in this dictionary:


  放在动词或形容词后，表示动作或变化已经完成：写完～。


付了: get it paid; 忘了: forgotten it. 


  还是付了吧，要不忘了: 
  
  
  Better get it paid (now), or you/I would/might have forgotten (to pay it).
  Better get it paid (now), in case you/I would/might have forgotten (to pay it).