If you wish to put the entire fee on your customer, remember that the customer must also pay the fee of collecting the fee itself.
r = 0.025 // (rate fee)
f = 0.25 // (flat fee)
O = 100 // original amount (the cost of some product)
Naive (wrong) approach:
R = O * r + f
= 100 * 0.025 + 0.25
= 2.75 // total fees
T = O + R
= 100 + 2.75
= 102.75 // total to pay by customer
The problem is that 2.75 is not what you are charged to make a 102.75 transaction. Instead you are charged 2.81875. Thus making you loose 0.07.
The correct approach is reached with some algebra:
R = (O + R) * r + f // total fees (identical to former, but taking itself into account)
R = (O + R) * r + f // subtract (O + R) * r
R - (O + R) * r = f // lift parenthesis
R - O * r - R * r = f // add O * r
R - R * r = f + O * r // add parenthesis
R(1 - r) = f + O * r // divide by (1 - r)
R = (f + O * r) / (1 - r)
R = (f + O * r) / (1 - r)
// total fees
R = (f + O * r) / (1 - r)
= (0.25 + 100 * 0.025) / (1 - 0.025)
= 2.82051282051
T = O + R = 100 + 2.82 = 102.82 // total to pay by customer
And we can verify the approach easily: 100 + 102.82 * 0.025 + 0.25 ≈ 102.82