The worldwide price to send an international postcard

I collected the price data from this post. Besides, I checked the postcards I received and added part of the data.

Here is the procedure to calculate the comparable price:
1. transform the price in different currencies into US dollars using the current exchange rate;
2. find the World Bank price level ratio of PPP (purchasing power parity) conversion factor to market exchange rate here for each country;
3. divide the price in USD (step 1) by the national price level (step 2).

If the price in Cuba is 0.85 CUP and 1 CUP = 0.038 USD, then Cuba has the lowest price so far, only 0.10 USD (purchasing power in the US), given the World Bank price level is right. If the price in Mozambique is 92 MZN and 1 MZN = 0.031, then Mozambique has the highest price, 5.22 USD PPP. As discussed, the prices of Italy and Poland are on the top, $2.69, $2.43 respectively. Note that the US price is $1.15.

Here is the world map, and the comparable price in USD is divided into four groups, <=1, 1-1.5, 1.5-2, >2, denoted by four increasing red colors.

Here is a zoomed European map.

You can find the raw data here. Please comment in the spreadsheet if the data are incorrect or to update the data.

Any suggestions or comments about the calculation are welcome!