ds[e_, th_]:= a^2r^2(P[e, th]^2)(P[e, th]+P[e, th]^(-1)-1+Cos[th]^2)/2

P[e_, th_]:=1/(1+(e/(m*c^2))(1-Cos[th]))

s=Integrate[ds[10^(-15), th],{th, 0, 2 Pi}]

1.66118*10^(-25)

ds2[th_]:=ds[10^(-15), th]/s

Integrate[ds[10^(-15), th], {th, 0, 2 Pi}]/s

1.

Integrate[ds[10^(-15), th]/s, {th, 0, 2 Pi}]

0.000217878