I'm in Canada. When I order from the US, I pay the bank rate of exchange, no extra fee.
We don't pay duty, but sometimes Customs makes us pay a $5 Cutsoms handling fee as well as GST (goods and services tax) on the value of the purchase.
Most of the time the parcel gets through without any additional charges, but it's a crap shoot. I ordered fabric last year that had to go in two flat rate packages. Mailed at the same time from the same company. One got through, the other came a week later, delayed at Customs while they added the fees. No rhyme or reason.
To order $25 one week and then $50 another would not be cost efficient due to shipping charges and the above mentioned possibility of getting a customs handling charge tacked on each parcel. As for it making a difference regarding Visa charges, I don't think it would matter.