Average of Percents, or Sum Total / Sum Total?

I have some data and need to know the average commission of all the items. For each row, I calculate the percent of items sent out. If I wanted to know, "What is the average commission", which is correct, $14.96$% or $12.89$%?

         Item Cost    Revenue     Commission
           1,000        289       28.90%
           2,991        190       6.35%
          10,400      1,050       10.10%
          20,000      2,900       14.50%
Totals    34,391      4,429       **14.96%**

The $14.96$% is calculated in Excel with:

=AVERAGE(28.90%, 6.35%, 10.10%, 14.50%) 

(Is this called an average of percents? Not sure what to call this).

Now, if I do 4429 / 34391 I get $12.88$%. Which average do I "want" to use to answer, "What's the average commission?" I think the SUM(Revenue) / SUM(Item Cost) is what I want, not the average of the percentages calculated, but don't know even what terms to use to look up what the difference is, or when to use one over the other. For the former, AVERAGE(), since that adds the percentages together, then divides by $4$, I don't think that is accurate for my purposes.