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No there is no simony ordinarily in marriage. No fees are absolutely required for marriage; that is, in case of true financial need, the sacrament would not and could not be denied due to inability to pay.
Again, however, I believe that this is a different scenario. There is no concept of needing to donate to the collection at mass to receive communion.
Yes, because why would there be? You seem to be assuming the lack of payment being assumed elsewhere implies that it is simony for marriage, when the reality is the opposite. There would be a much stronger argument for simony if you had to give to the collection to receive communion.
The reality of why there is no payment expected there is a mix of custom and practicality. It's not practical to inspect in most cases who gives and doesn't on a regular basis to determine who would be eligible (including those whose need prevents). More specifically, communion is ordinarily received in the context of liturgies the priest prepares for and celebrates as a course of his duty. He is thus paid to do so. Marriages are extra liturgies, which require extra work, namely preparing the couple, writing special sermons, familiarizing oneself with special texts, etc and for these he is entitled to some honorarium, but he may not require one as a condition to receive it. Note well that in absence of ability to pay, he must still perform a marriage (Assuming requirements are met) but he is under no obligation to provide a choir, flowers, etc.