The P515 can be recorded without hardware (just an Audio/USB link), the laptop mic, without hardware too. Then a virtual mixer like Banana Voicemeter may do the job.
The audio interface may be useful with an higher grade mic. But if the mic is only used to introduce piano-solo records, the laptop mic will be fine. A USB mic like the Behringer C-1U may be an option.
Note: for piano/voice record, a DAW may be useful and such a software use one audio interface at once. Then a 4 inputs interface may be handy. (Focusrite 4i4, Steinberg UR44…) The P515 is stereo: 2 inputs. And there are no 3 inputs interfaces !