It is not very clear what exactly is meant by "double conversion". You may be referring to cases when a foreign store charges a fee in rubles.
Usually, the store offers you the choice of paying in rubles or hard currency. In this case, you always need to choose the last option, because the exchange rate of a Russian bank in 99.99% of cases will be more profitable for the buyer than the rate of a foreign store. This difference will often be very significant. If the rate in your bank turns out to be less profitable than in the store, then you should think about changing such a bank (seriously).
Online or offline does not matter. Amazon offers to select the payment currency on the checkout. For the sake of interest, you can select the ruble as the currency of payment and see at what rate it is proposed to convert the check amount.
When making payments with a card abroad, inscriptions with currency options often appear on the payment terminal in the store. If you are not sure which buttons to press, tell the seller "I'd like to pay in US Dollars or Euro".
Small addition. Probably, in the case of online stores, this does not happen often, but offline stores abroad (especially in tourist places) sometimes offer a choice of several payment currencies. It can be euro, dollar, local currency and ruble. In this case, the best option is the local currency (whatever it is). The reason is the same - always the overpriced "shop" rate for a tourist in comparison with the rates of your servicing bank.
Overseas ATMs (especially in countries outside the Eurozone and the USA) can when withdrawing local currency from your card, offer to choose the exchange rate at which to carry out the operation. Let's say you shoot coons in Croatia or dirhams in the UAE. At the same time, the ATM offers you "guaranteed" and "non-guaranteed" exchange rates. The "guaranteed" rate is generally understood to mean the exchange rate of the local currency for USD / EUR. To compare the two rates, you need to convert (1) the amount indicated on the screen in dollars / euros into rubles and (2) the amount of withdrawal in local currency into rubles and compare in which case the ruble amount is less (more profitable). In practice, this is quite difficult to do, especially if there is a queue at the ATM (you need to know the current rates of your bank for dollars / euros and local currency (!) And be able to quickly calculate in your head). As you can guess, local banks thus "parasitize" on tourists' fear of the word "non-guaranteed", so this course is worth choosing. Based on my own experience, I can say that the "guaranteed" course is likely to be 10-15% less profitable.
I think it is best to ask your bank's support, Christina)
The fact is that the bank decides which currency or currencies to settle with the payment system. Each bank has its own list of currencies for such settlements.
In most cases, the billing currency (settlement currency) between the Visa payment system and the bank is dollars; between MasterCard and the bank - either dollars or euros if you are in the eurozone (states whose official currency is the euro).
In general, if the bank assumes the conversion costs when the account (card) currency matches and operation currency (price tag of goods, services), when traveling to the eurozone, it is more profitable to use a card with an account in euros, and when traveling to the dollar zone or to countries with currencies other than dollars and euros, with an account in dollars. The same is true for online stores located in the corresponding locations.
I repeat again - you should clarify this information (settlement currency, commission for conversion) with your bank + consider the advantages of opening a multicurrency card.